Adding a release function for ThreadLocalPtr.

Adding a release function for ThreadLocalPtr.

Summary:
ThreadLocalPtr manages the lifecycle of the object that is
stored with it. We have a use case where we sometimes want to transfer ownership
of the stored object to another thread by wrapping them with
unique_ptrs. Adding a release function, similar to to the
unique_ptr::release is the cleanest way for us to transfer ownership.

Test Plan:
I can do some on off testing using a command line tool, but I
was wondering about how to add some unit tests. Not sure when the folly
unit tests were.

Reviewed By: njormrod@fb.com

FB internal diff: D1321588