[WIP] Remote sync revamp

[wooj2]

Don't review this. This is actually four separate commits I will be splitting up in the very near future.

Uploading this to a branch in the event my computer gets hit by a bus.

Notable updates:

• remote sync engine was moved to a serial statemachine
• handle a sendCompletion (failure) in order to kick the state machine to an errored/clean up restart
• Added cancel interface to cancel event reconciliation queue, model reconciliation queue, listeners, mappers, etc.
• Added sending connection information from API in order to decide when subscribeInitialization succeeds (e.g., if we can't make a subscription connection, then we shouldn't be doing an initialSync)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[kneekey23]

guessing this gets called multiple times and you only want it called once? can you removeDuplicates() on the sink that calls this

[wooj2]

Great callout -- and actually this may not be needed at all

What I was actually trying to protect against is not multiple calls on a single subscription - but I think there was a bug in my clean up logic, where i was only pausing the queue (rather than destroying it), which lead to multiple onReceiveCompletions.

[kneekey23]

lol

[wooj2]

yeah, whooops that was removed . in a subsequent pr :P

[wooj2]

Closing this pr.. as i have split this PR into 4 distinct PRs:
#321 - migrating to a state machine
#322 -added retry logic to sync engine, without canceling downstream async operations
#323 - added in the cancelling interface
#324 - cancelling the downstream operations & wait for subscription to init.