Private class methods which write the logs and write the binaries
have been added to Profile. The binaries are able to be used
by various visualisation tools.

Make the profiler output binaries (which can be used to visualise the data)

See merge request !1082

.gitlab-ci.yml: Add overnight tests logs to artifacts.

See merge request !1084

This patch by itself fixes #868

Follow up of last commit which uses exclusive locking to
protect bzr operations instead.

With get_element_state(), you need to invoke BuildStream once
for every element state you want to observe in a pipeline.

The new get_element_states() reports a dictionary with
the element state hashed by element name and is better to use
if you have more than one element to observe the state of.

This causes multiple source instances to interact with the same
backing data store at the same time, increasing the likelyhood
of triggering issues around concurrent access.

This more reliably triggers issue #868