Fix potential race condition with delete.  Document ACID.

Summary:
Fix race condition that can occur with deletes where two concurrent queries change visibility or delete the row.  This is fixed by acquiring a row lock upon the initial read of the row.

Improve documentation of how we maintain consistency of counts with
InnoDB transactions in other cases.

Test Plan: Ran MySQL tests

Reviewers: dhruba, emayanke, MarkCallaghan

Reviewed By: MarkCallaghan

Differential Revision: https://reviews.facebook.net/D10659