Summary:
Move async socket to folly.

Changes:
* Made an AsyncSocketException type instead of TTransportException: Some of the exceptions didn't fit nicely in to std::exception types (like TIMED_OUT).  There are some wrappers in thrift/lib/cpp/async to convert back to TTransportException, so all existing code still compiles.
* Moved read/write callbacks out of AsyncTransport: filters are going to want to do the read/write stuff separately (see revproxy/tunnel/filters, and discussions in D1483148).

Test Plan:
fbconfig -r thrift; fbmake runtests
contbuild should catch everything else - exception types shouldn't change for existing code

Reviewed By: dcsommer@fb.com

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

FB internal diff: D1587625