Before these fixes, it was possible to see errors on new RPCs after a
connection began draining, and before establishing a new connection.  There is
an inherent race between choosing a SubConn and attempting to creating a stream
on it.  We should be able to avoid application-visible RPC errors due to this
with transparent retry.  However, several bugs were preventing this from
working correctly:

1. Non-wait-for-ready RPCs were skipping transparent retry, though the retry
design calls for retrying them.

2. The transport closed itself (and would consequently error new RPCs) before
notifying the SubConn that it was draining.

3. The SubConn wasn't synchronously updating itself once it was notified about
the closing or draining state.

4. The SubConn would go into the TRANSIENT_FAILURE state instantaneously,
causing RPCs to fail instead of queue.