Move thrift/lib/cpp/async to folly.

Move thrift/lib/cpp/async to folly.

Summary:
Move the minimum amount of stuff and still have everything compile.  Would like to move TAsyncSocket/ServerSocket/SSL/UDP eventually, but not this round.

thrift async is used very widely now: thrift, proxygen, newer mysql async interface, even trying it out in memcache.  A common complaint is that it doesn't get wide enough notice under thrift/, so let's move it to folly/.  After moving TAsyncSocket, both HHVM and proxygen could avoid a dep on thrift as well.

Changes:
* mv files to folly/io/async
* remove 'T' prefix on classes/filenames
* change namespace to 'folly'
* remove any thrift references.

Tried this before in D470080, this time doesn't attempt to abstract libevent

@override-unit-failures

Test Plan:
fbconfig -r thrift; fbmake dev.
Will iterate on any other contbuild failures

Reviewed By: pgriess@fb.com

FB internal diff: D1195393