Make RequestChannel::sendRequestSync work when not in running event base thread
Make RequestChannel::sendRequestSync work when not in running event base thread

Summary:
Title.

(Note: this ignores all push blocking failures!)

Reviewed By: yfeldblum, mnv104

Differential Revision: D14839411

fbshipit-source-id: dd966426c9c73afafd3469ff2ac689bf8d5806ba

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