[Reviewer: Alex] UTs run much faster if we don't pause for 1s every time we destroy a Sig...

...nalHandler

More fixes addressing https://github.com/Metaswitch/sprout/issues/588. This brings it down from 24s to 10s, and I have another change in Sprout bringing it down to 6s.

Tested by running the UTs, but not live - I don't think that's worthwhile as we only destroy the updaters on program termination.
Good spot - I assumed from the initial underscore that this was a member variable, but it's actually a global. I need do figure out how we can create and destroy multiple BgcfServices throughout the UTs, and have a single global signal handler, without having this one-second pause.