Continuation termination callback support

[bretambrose]

• adds a termination callback for continuations. This gives us a safe, guaranteed (regardless of state) timepoint after which we know no further callbacks are possible for binding object lifetime management.
• adds a termination callback for connections. This gives us a safe, guaranteed (regardless of state) timepoint after which we know no further callbacks are possible for binding object lifetime management.
• relaxes the server-side constraint around stream id lookup for incoming messages. A terminate-stream-flagged message with no corresponding continuation is no longer a protocol error in order to support simultaneous close. This probably should have been done three years ago when Relax protocol constraint to work around client-side race condition #83 was applied.
• Fixes a continuation table race condition where we could be iterating the continuation table while a concurrent execution path attempts to remove an entry
• Eventstream connections now select and track their event loop before the connection attempt. This allows us to reason about whether or not a callback can concurrently overlap binding logic (ie, if we're in the event loop, it cannot).

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

[sbSteveK]

Kinda unrelated to this PR but while we're here we may as well fix this comment:
Line 75 in event_stream_rpc_client.h * but it is still open, you must call aws_aws_event_stream_rpc_client_close()
should probably be pointing to aws_event_stream_rpc_client_connection_close

[sfod]

LGTM