WIP: Continued work on improving BuildStream messaging API

Description

Continued work on @tlater's !527 (closed)

This brings the changes up to date with master.

Changes proposed in this merge request:

• Bring !527 (closed) up to date with master
• Add message helpers to job, elementjob (specialised for scheduler kwarg)
• Add message helpers to queue to avoid self._scheduler.context.info()
• Longest call to a log function used is now self._context.info

This works towards: #456 (closed)