statsd: Rename gearman.py to statsd_gearman.py

Trying to import from the top-level gearman package was not
working because Python would attempt to import from the local
level file first, since it, too, was named gearman.py.

Change-Id: I535fb82490c5d7a2e4e8d7a2a2c82ce1ccb06b61