Udp Acceptor

Udp Acceptor

Summary:
major changes:

1) ServerSocketFactory and AsyncSocketBase to abstract the differences between UDP and TCP async socket.  Could possibly push some of this to the sockets themselves eventually
2) pipeline() is a pipeline between accept/receive of a UDP message, and before sending it to workers.  Default impl for TCP is to fan out to worker threads.  This is the same as Netty.  Since we don't know if the data is a TCP socket or a UDP message, it's a void*, which sucks (netty uses Object msg, so it isn't any different).

Test Plan: Added lots of new tests.  Doesn't test any data passing yet though, just connects/simple receipt of UDP message.

Reviewed By: hans@fb.com

Subscribers: alandau, bmatheny, mshneer, jsedgwick, yfeldblum, trunkagent, doug, fugalh, folly-diffs@

FB internal diff: D1736670

Tasks: 5788116

Signature: t1:1736670:1424372992:e109450604ed905004bd40dfbb508b5808332c15