msg/async: fix is_queued() semantics

msg/async: fix is_queued() semantics

Before AsyncConnection was split into two classes as part of the
multi-protocol refactor, we only had AsyncConnection::is_queued().
It checked both out_q and outcoming_bl because out_q was part of
AsyncConnection.

out_q is now part of ProtocolV1.  AsyncConnection should no longer be
concerned with out_q, only with outcoming_bl.  Checking whether out_q
is empty in _try_send() is particuarly wrong because if the write is
fininished (i.e. outcoming_bl is empty) but out_q has something in it,
the write callback isn't invoked.

Although probably not strictly necessary, this commit preserves the
semantics of connection->is_queued() in Protocol.cc.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>