Normalize structured logging data

Summary:
Refactored serialization of structured JSON logging to use Python's
native support for "extra" logging.LogRecord attributes over a
specialized log.Message class, and to include as much LogRecord data
as the class will allow us to reconstruct on the consuming end.

This should provide a more consistent interface for log records
regardless of where they originate, making implementation of T115137
more straightforward.

Test Plan: Run tox or unittests in tests/log_test.py and tests/ssh_test.py.

Reviewers: demon, thcipriani, mmodell

Reviewed By: thcipriani, mmodell

Maniphest Tasks: T115137

Differential Revision: https://phabricator.wikimedia.org/D15