When we hit the limit on the number of workers the proxy thread would
stop processing incoming messages, sending it into an infinite loop of
death. The check was supposed to use `active_workers()` rather than
`workers.size()`, but even that isn't quite right: we want to *always*
pull all incoming messages off and queue them internally since different
categories have their own queue sizes (and so we have to pull it off to
know whether we want to keep it -- if spare category queue room -- or
drop it).
We really don't *ever* want send to block, no matter how it is called,
since the send is always in the proxy thread. This makes the actual
send call always non-blocking, and adds callbacks that we can invoke on
send failures: either on queue full errors (which might be recoverable),
or both full queue and hard failures (which are generally not
recoverable). These callbacks are both optional: they have to be passed
in using `send_option::queue_full` (if you just want queue full
notifies) or `send_option::queue_failure` (if you want queue full
notifies *and* other send exceptions).
lokimq.cpp and lokimq.h were getting monolithic; this splits lokimq.cpp
into multiple smaller cpp files by logical purpose for better parallel
compilation ability. It also splits up the lokimq.h header slightly by
moving the ConnectionID and Message types into their own headers.
