Implement job queue and thread pool for I/O to files and plugins

[aremmell]

If a write is to a network drive, or a plugin makes a network call as part of its behavior, the thread that called into libsir could stall.

Instead of letting that happen, stick the write job into a queue that is picked up by a pool of threads which do nothing except wait for a job to be available, process it, and resume waiting.

[aremmell]

Implemented in thread-pool branch. More testing and consideration must go into it before putting it to use. As of now, not even sure it is a solution to the original issue:

A thread queue with 4 threads calling into a plugin or network file that blocks for 60 seconds will immediately be fully occupied - and therefore:

• needlessly growing the queue of incoming jobs in RAM
• will never catch up, because the log messages will continue to come in faster than the messages can be dispatched, even by 4 separate runners.

Perhaps we should stash away output meant for destinations we know to currently be blocking/taking too long to write, then when their responsiveness is better, pull it out from a file and write it to the dest.

[johnsonjh]

This should also be used for syslog and systemd logging. The possibility of blocking here is low but real.