Add PhabricatorWorker->log()

Summary:
Ref T2852. Add a log() method to PhabricatorWorker to make debugging easier.

I renamed the similar Drydock-specific method.

Test Plan:
Used logging in a future revision:

...
<<< [36] <http> 211,704 us
Updating main task.
>>> [37] <http> https://app.asana.com/api/1.0/tasks/6153776820388
...

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6296