Make internal Realtime APIs consistent

[lawrence-forooghian]

From InternalRealtimeClientProtocol introduced in #244:

Note that the API of this protocol is not currently consistent; for example there are some places in the codebase where we subscribe to Realtime channel state using callbacks, and other places where we subscribe using AsyncSequence. We should aim to make this consistent (…)

┆Issue is synchronized with this Jira Task by Unito

[lawrence-forooghian]

No longer relevant after #247, which removes the use of AsyncSequence for receiving ably-cocoa events.