merge wangle/Executor.h and experimental/wangle/concurrent/Executor.h

merge wangle/Executor.h and experimental/wangle/concurrent/Executor.h

Summary:
the one in concurrent/ is a bit more generic, so I kept that as Executor and renamed the existing one ScheduledExecutor

because Hans is surfing I took the liberty of renaming Action->Func as an alias for std::function<void()>, because I think it's more reflective
also kept the version of add() that doesn't force rvalue-reference as it's more user friendly and probably not less performant in common cases (insert reference to "want speed? pass by value" here)

Test Plan: compiled some major relevant bits, will let contbuild show me anything I missed

Reviewed By: hans@fb.com

Subscribers: trunkagent, rushix, fbcode-common-diffs@, fugalh, msk, njormrod

FB internal diff: D1591237

Tasks: 5279196