zuul: debug gear.Server

The internal class gear.Server logs interesting message at debug level,
log them down to /var/log/zuul/gearman-server-debug.log with a few days
of archives.

While at it add vim modeline and some comments to let me better read the
file.

Applied in production manually.

Change-Id: Idda66be684a568050cae2f4fb9739d09564b48a2