Summary: Fixes T9715. Adds a MySQL-based lock to ensure that schema migrations are not applied on multiple hosts simultaneously.
Test Plan: Ran `./bin/storage upgrade` concurrently. One invocation was successful whilst the other hit a `PhutilLockException`.
Reviewers: #blessed_reviewers, epriestley
Subscribers: Korvin
Maniphest Tasks: T9715
Differential Revision: https://secure.phabricator.com/D14463
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