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: b48fe4683165

Our application can't terminate since this change was committed. 100% reproducible.

Reviewed By: WillerZ

Differential Revision: D26621716

fbshipit-source-id: 571ecd9a6f5286469877bf510f1f55b036df9217

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