Apple, in particular, often fails tests with an address already in use
if attempt to reuse a port that the process just closed, because it is a
wonderful OS.
This allows mixing some outside task into the lokimq job queue for a
category (queued up with native LMQ requests for that category) for use
when there is some external process that is able to generate messages.
For example, the most immediate use for this is to allow an HTTP server
to handle incoming RPC requests and, as soon as they arrive, inject them
into LokiMQ's queue for the "rpc" category so that native LMQ rpc
requests and HTTP rpc requests share the same thread pool and queue.
These injected jobs bypass all of LokiMQ's authentication and response
mechanisms: that's up to the invoked callback itself to manage.
Injected tasks are somewhat similar to batch jobs, but unlike batch jobs
the are queued and prioritized as ordinary external LokiMQ requests.
(Batch jobs, in contrast, have a higher scheduling priority, no queue
limits, and typically a larger available thread pool).
Jason Rhinelander