[Reviewer Andy] Issue739sprout

Andy

As discussed, can you review my fix to https://github.com/Metaswitch/sprout/issues/739.  Fix is probably a strong word!  It doesn't fix the underlying issue in ttlcache (although I have included the attempted fix I made there, which does fix another bug), but changes the blacklist to use a simpler data structure.

I've tested by running the Sprout UT (which include resolver blacklist tests) and running the stress test that was previously crashing for 20+ hours with no crashes.

Mike
Is the blacklist supposed to be a delta time?  If so we should be using the monotonic clock, not the system clock (which is subject to drift and leaps).