Summary: Since loopKeepAlive() is always used from the EventBase thread, there's no need for the overhead of an shared_ptr (and therefore, an atomic ref counter); we can get away without thread safety. This also allows us to discard the (sometimes incorrect) optimization of not returning a handle when it appears the loop will continue running anyways

Reviewed By: andriigrynenko

Differential Revision: D3375503

fbshipit-source-id: 474e4fcf992bdc4fcca9370d3c57bdcc4e042386