Dead letters containing remote envelopes handled correctly #2959

 - New DeadLetter class for handling remoting specific envelopes
 - Fixed error handling of name lookups
 - Name lookup is now handled via futures (future refactor opportunity)
Merge pull request #1077 from drewhk/wip-2959-cluster-deathwatch-fix-drewhk

Dead letters containing remote envelopes handled correctly #2959