Implementing bb's standard db access pattern for last-N-failed builder

MTRLogObserver is deprecated starting with Buildbot 3.3.0.
Where possible, without breaking existing builders, we need to
move away from using MTR related classes and methods.

In this case, subclassing MTR was not even needed for getting the test failures.

This patch is implementing the standard db access pattern of Buildbot.
See:
https://buildbot.readthedocs.io/en/latest/developer/database.html#module-buildbot.db.pool

Although the recommended way is to retrieve data through the data api
our use case is special enough to make an exception, given that
the test_ table schemas will become in-house maintained when we migrate
to newer versions of buildbot.