Summary:
[Folly] Split out the global `ShutdownSocketSet` singleton into its own library to keep the dependencies of `ShutdownSocketSet` lighter-weight.

In general, globals should be an opt-in sort of thing anyway.

Reviewed By: elsteveogrande

Differential Revision: D6790009

fbshipit-source-id: c68fa2bbaeaf3ff205e0c3b1c739d7561e940e64