Overhaul Message API

_context.py: message API standardisation, all messages should
come through context. Added skipped() for MessageType.SKIPPED.
Also added context.report_unhandled_exceptions() to standardise
catching unhandled exceptions and sending a BUG message.

job.py: Add context attribute for referencing the context during messaging
calls. Messaging helpers added to use context helpers with task_id kwarg.
_child_action() has been overhauled to no longer use parts of
timed_activity, it now takes advantage of timed_activity and helper
functions.

queue.py: Now takes advantage of context helper functions and uses
report_unhandled_exceptions instead of MessageType.BUG in _job_done().

Others: Removing redundant imports and changing to use context message
API.