feat: RedisServerAffinity RFC: Lock one document to one server

[mattkrick]

Follow-up to #983

Yes, in general the way to go is make sure that users working on the same document are connected to the same server
#983 (comment)

This extension guarantees that 1 document is only open on a single server at any given time. If a client attempts to access a document that is already opened on a different server, their messages get proxied to that server via Redis.

If a server needs to lock a document (e.g. before calling openDirectConnection), it can call the lockDocument method. Alternatively, it may use this extension like an event emitter that emits to the owning server via customEvents and the handleEvent method.

May be of interest to @0xb4lamx @janthurau