Generate updated mongoid.yml config file

I generated the file with `rails g mongoid:config` and then copied the
custom options from our previous config.

Notable differences:

* The `safe: true` option has been removed. The new `w: 1` option
  provides the same behaviour by default. From the Changelog for v4[1]:

    > Mongoid now defaults all writes to propagate (formerly "safe
    > mode") and now has different propagate semantics:
    >
    > { w: -1 }: Don't verify writes and raise no network errors.
    > { w: 0 }: Don't verify writes and raise network errors.
    > { w: 1 }: Verify writes on the primary node. (default)
    > { w: n }: Verify writes on n number of nodes.
    > { w: "majority" }: Verify writes on a majority of nodes.

* The `consistency: strong` option has been replaced by `read: primary`
  in the test and production sessions. You can see from the comments in
  this diff that these are equivalent.

* The `use_activesupport_time_zone`, `refresh_interval` and
  `refresh_mode` options have been moved to the "Configure Mongoid
  specific options" key.

[1]: https://github.com/mongodb/mongoid/blob/master/CHANGELOG.md

config/mongoid.yml

@@ -9,38 +9,63 @@ development:
       # Provides the hosts the default session can connect to. Must be an array
       # of host:port pairs. (required)
       # hosts:
-      #  - localhost:27017
+      #   - localhost:27017
       uri: 'mongodb://localhost/govuk_content_development'
       options:
-        # Change whether the session persists in safe mode by default.
-        # (default: false)
-        safe: true
+        # Change the default write concern. (default = { w: 1 })
+        # write:
+        # w: 1
 
-        # Change the default consistency model to :eventual or :strong.
-        # :eventual will send reads to secondaries, :strong sends everything
-        # to master. (default: :eventual)
-        # consistency: :eventual
+        # Change the default consistency model to primary, secondary.
+        # 'secondary' will send reads to secondaries, 'primary' sends everything
+        # to master. (default: primary)
+        # read: secondary_preferred
 
         # How many times Moped should attempt to retry an operation after
-        # failure. (default: 30)
-        # max_retries: 30
+        # failure. (default: The number of nodes in the cluster)
+        # max_retries: 20
 
         # The time in seconds that Moped should wait before retrying an
-        # operation on failure. (default: 1)
-        # retry_interval: 1
-  # Configure Mongoid specific options. (optional)
-  options:
-    # Configuration for whether or not to allow access to fields that do
-    # not have a field definition on the model. (default: true)
-    # allow_dynamic_fields: true
+        # operation on failure. (default: 0.25)
+        # retry_interval: 0.25
+
+        # The connection pool size per-node.  This should match or exceed the
+        # number of threads for a multi-threaded application. (default: 5)
+        # pool_size: 5
+
+        # The time in seconds that Moped should wait for the pool to provide
+        # an available connection.  This number should probably remain at the
+        # default, unless for some reason you absolutely need to limit the
+        # pool_size, as this wait is only used when the pool is saturated.
+        # (default: 0.5)
+        # pool_timeout: 0.5
+
+        # The time in seconds before Moped will timeout connection and node
+        # operations. (default: 5)
+        # timeout: 5
+
+        # The amount of time in seconds between forced refreshes of node
+        # information including the discovery of new peers. (default: 300)
+        # refresh_interval: 300
 
-    # Enable the identity map, needed for eager loading. (default: false)
-    # identity_map_enabled: false
+        # The amount of time in seconds that a node will be flagged as down.
+        # (default: 30)
+        # down_interval: 30
 
+        # Whether connections should use SSL. (default: nil/false)
+        # ssl: false
+
+        # Whether Moped will use the existing seeds/nodes to find other peers.
+        # (default: true)
+        # auto_discover: true
+
+
+  # Configure Mongoid specific options. (optional)
+  options:
     # Includes the root model name in json serialization. (default: false)
     # include_root_in_json: false
 
-    # Include the _type field in serializaion. (default: false)
+    # Include the _type field in serialization. (default: false)
     # include_type_for_serialization: false
 
     # Preload all models in development, needed when models use
@@ -58,11 +83,7 @@ development:
     # existing method. (default: false)
     # scope_overwrite_exception: false
 
-    # Skip the database version check, used when connecting to a db without
-    # admin access. (default: false)
-    # skip_version_check: false
-
-    # User Active Support's time zone in conversions. (default: true)
+    # Use Active Support's time zone in conversions. (default: true)
     # use_activesupport_time_zone: true
 
     # Ensure all times are UTC in the app side. (default: false)
@@ -76,7 +97,7 @@ test:
       #   - localhost:27017
       uri: 'mongodb://localhost/govuk_content_manuals_publisher_test'
       options:
-        consistency: :strong
+        read: primary
         # In the test environment we lower the retries and retry interval to
         # low amounts for fast failures.
         max_retries: 1
@@ -88,7 +109,8 @@ production:
       uri: <%= ENV['MONGODB_URI'] %>
       options:
         logger: false
-        use_activesupport_time_zone: true
-        refresh_interval: 120
-        refresh_mode: :sync
-        consistency: :strong
+        read: :primary
+  options:
+    use_activesupport_time_zone: true
+    refresh_interval: 120
+    refresh_mode: :sync