Provide logging API to bjam engine

[stefanseefeld]

With the separation between backend and frontend, it becomes hard to collect a unified stream of logs including every aspect of the work flow. It should be possible to channel log output from both the engine and the Python frontend through the same transport, to:

• make it easier to trace things
• have uniform formatting
• have a common way to configure logging

(this could mean that either bjam will send its own output to a callback that could be set by the Python frontend, or it could mean that the Python frontend sends its own output to bjam)

[stefanseefeld]

This is obsoleted by the new asyncio-based scheduler.