BUG-8403: guard against ConcurrentModificationException

BUG-8403: guard against ConcurrentModificationException

Using TransmitQueue.asIterable() offers slight advantage of not
dealing with a big list, but exposes us to the risk of the Iterable
being changed.

The point missed by the fix to BUG 8491 is that there is an avenue
for the old connection to be touched during replay, as we are
completing entries, for example reads when we are switching from
remote to local connection. In this case the callback will be invoked
in the actor thread, with all the locks being reentrant and held,
hence it can break through to the old connection's queue.

If that happens we will see a ConcurrentModificationException and
enter a buggy territory, where the client fails to work properly.

Document this caveat and turn asIterable() into drain(), which
removes all the entries in the queue, allowing new entries to be
enqueued. The late-comer entries are accounted for when we set the
forwarder.

Change-Id: Idf29c1e565e12aaed917ac94c21c552daf169d4d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>