Summary: Added EventListener::OnTableFileCreationStarted. EventListener::OnTableFileCreated will be called on failure case. User can check creation status via TableFileCreationInfo::status.

Test Plan: unit test.

Reviewers: dhruba, yhchiang, ott, sdong

Reviewed By: sdong

Subscribers: sdong, kradhakrishnan, IslamAbdelRahman, andrewkr, yhchiang, leveldb, ott, dhruba

Differential Revision: https://reviews.facebook.net/D56337