Add ability to open a synced realm without sync configuration.

[leemaguire]

Add ability to open a synced realm without a flexible sync configuration.

auto local_config = realm::db_config();
local_config.set_path(path);
local_config.enable_forced_sync_history(); // allows a synced realm to be used with a local config.

[dianaafanador3]

LGTM! Just a few comments and some minor questions

[adamski]

@leemaguire I assume in order to move the local realm to be synced, one would need to explicitly set the path to be the same in both cases, rather than use the default path given when setting it up via an app and user?