Replaced beanstalkd with ZeroMQ PUSH/PULL sockets.

Also added (back in) an actual connection pool for the workers (got
broken when switching from eventlet to gevent).

Tweaked logging.

Added last-byte-latency markers to STDERR output during benchmark runs
for operation types which don't have first-byte-latency (CREATE/UPDATE).

The master can take the ZMQ bind IP as an argument (instead of
having 0.0.0.0 hard-coded).  That way, if someone wants to be all
private and use 127.0.0.1 or some other single interface, they can.

Added convenience-spawning/reaping of ssbench-workers (but only for the
local host, of course).

Updated documentation for ZMQ change.

Fixes #10.
Fixes #18.
Fixes #20.