unix_msg: introduce send queue caching

unix_msg: introduce send queue caching

This introduces caching of unix datagram send queues. Right now send
queues are only created for peers if the channel to the peer is full and
a send reported EWOULDBLOCK.

At this stage, performance will actually be slightly worse, because now
if there's a cached queue for a peer without queued messages, we don't
attempt direct send anymore until the send queue is removed from the
cache.

The next commit will modify unix_msg to always create a send queue with
the datagram socket in connected mode and again attempt an non-blocking
send on the connected socket first. Then only if that returns
EWOULDBLOCK, the send has to go through the threadpool.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>