[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
I'd have preferred to keep the typedef (so we don't have code that looks like `std::map<AddrInfo, time_t>::iterator` later.