Integrate log handlers into zmq io loop.

Currently, circus polls logging handlers once every second.  This is inefficient,
and easily stalls processes that emit more log messages in one second than fits
into the systems pipe buffer (typically 4-64 KB).  I have actually seen this
happening with BrightcoveOS's Diamond server.

This change integrates log handling into the zmq ioloop, which should handle any
amount of log traffic fairly and without arbitrary delay.