Move executor to local var immediately in doCallback
Move executor to local var immediately in doCallback

Summary:
[Folly] Move executor to local var immediately in `doCallback`, and always move it into the callback.

(Note: this ignores all push blocking failures!)

Reviewed By: andrewcox

Differential Revision: D15532854

fbshipit-source-id: 971a6a64ad3c47c9f5e9f75907a568f9d59b1000

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