Summary: This diff adds UpdateableSharedPtr and FastUpdateableSharedPtr classes to folly. They are efficiet RCU-style holders for a shared_ptr. See the comment at the top of UpdateableSharedPtr.h for a more detailed description and benchmark results.

All the added files are copied from logdevice code with (almost) no changes (`logdevice/common/` on branch `origin/logdevice-latest`). We've been using them in logdevice for a few months. D1821723 introduced it.

Reviewed By: @​bmaurer

Differential Revision: D1919702