End of session reports

Currently we display status information about the queues in operation and the total elements of the pipeline vs how many elements were selected for processing in the status area, but these dont end up in a master build log.

Also it will be nice for the user to see something informative when buildstream exits about what was done.

This should essentially have overall information such as:

• What main action was performed (track, fetch, build)
• Total pipeline elements
• Elements processed

And information pertaining to each queue, such as:

• Elements which were successfully processed
• Elements which were skipped
• Elements which failed