folly::FunctionScheduler: replace std::function w/ folly::Function

folly::FunctionScheduler: replace std::function w/ folly::Function

Summary:By using folly::Function instead of std::function to internally store
functions in FunctionScheduler, this diff allows users to pass
non-copyable lambdas to FunctionScheduler::addFunction.

All exisiting unit tests still pass. Also, passing std::function to
addFunction still works, as the std::function will be implicitly
converted (i.e. wrapped) in a folly::Function. However, this implies
a performance penalty.

Reviewed By: fugalh

Differential Revision: D3092944

fb-gh-sync-id: 1e8081e70d4717025f2eadbb6b6da173460d4373
fbshipit-source-id: 1e8081e70d4717025f2eadbb6b6da173460d4373