Back out "Switch SerialExecutor to default to the immutable global executor"
Back out "Switch SerialExecutor to default to the immutable global executor"

Summary:
Original commit changeset: c9b68c6b9fac
Backout: D26916419 (https://github.com/facebook/folly/commit/b9f3d9be479d4dccdc8e5094933229e676308872)

Differential Revision: D28532140

fbshipit-source-id: 15e5eb1262234039387bfda36d280391f52cc3b2

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly