When a conductor is running it is quite useful to be able to how
long each engine step takes. To enable this information being
output, add a notifier to the base conductor and use it in the
blocking conductor to emit events around engine activities. This
makes it possible to track the timing (or other information that
can be gathered from these events) in a non-intrusive manner.
In the `99_bottles.py` demo we also now use this to be able to
easily see what the conductor is actively doing (without having
to enable the more verbose DEBUG level logging).
Change-Id: Ifd8ff38f82fc8135fe5fec4c8e41f0e06f4fdee3
63c6730248