Summary:
Move shutdownsocketset to folly, since it is a dep of the asyncsockets

Previoulsy tried moving it in to the server directly: D1583629, but had issues - close(fd) is called before the error callback, so we can't remove the fd before the close, which is essential to it working properly.

Just move it to folly instead.

Test Plan: fbconfig -r thrift/lib/cpp thrift/lib/cpp2; fbmake runtests

Reviewed By: dcsommer@fb.com

Subscribers: mshneer, trunkagent, fugalh, jsedgwick, doug, alandau, bmatheny, njormrod

FB internal diff: D1594950