Broadcast session and room changes from sync worker (no encryption yet)

[psrpinto]

Broadcast session and room-level sync changes from worker to main thread(s).

Encrypted rooms are not yet implemented, that will be addressed in the next PR.

Summary of changes

1. Introduce a SendQueuePool which allows us to listen to newly-generated events across all rooms. These events are then propagated to the worker. Since "pending events" are handled as part of the sync process, the worker needs to be aware of them so that the sync properly removes them as pending.
2. Serialize and deserialize sync changes on the worker and in the main thread, respectively.

Screen recordings

Screen.Recording.2023-04-06.at.16.19.18.mov

Screen.Recording.2023-04-06.at.16.53.43.mov

[psrpinto]

I accidentally merged #15 to the wrong branch, this PR is the same as that one but targetting master.