Summary: [Folly] Split KeepAlive concepts of dummy and alias. A dummy KeepAlive is one for an executor which does not actually support keep-alive semantics. An alias KeepAlive is one for which there is another KeepAlive, with a surrounding lifetime, to the same executor.

Reviewed By: andrewcox

Differential Revision: D15683241

fbshipit-source-id: a5809b06c90ed4a655a6973fac67137b5e1981dc