Review the need for the channels property in node_spec()

[aramallo]

In V5 we have an updated node_spec

-type node_spec()           ::  #{
    name := node(),
    listen_addrs := [listen_addr()],
    channels := #{channel() => channel_opts()}
}.

parallelism is now a per channel property.

We need to review the need for channels in this structure.

All nodes in a partisan cluster should have the same channels configuration, so when using a discovery strategy e.g. DNS we get nodename and list_addrs and we append the local channels config to create the node_spec(). If a node is wrongly configured we will have two different node_spec() objects for the same nodename.

The proposal would be to remove channels from the spec and just get it from the config partisan_config:channels().

-type node_spec()           ::  #{
    name := node(),
    listen_addrs := [listen_addr()],
}.

This also makes the membership CRDT and the gossip payload smaller