Quiet utf8mb4 warnings when establish MySQL connections

Summary: Fixes T6228. Ref T1191. Some versions of things spew warnings on these calls; I can't reproduce it locally but @fabe reports this resolving it for him in T6228.

Test Plan: Still works fine locally, but no surprises there.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: fabe, epriestley

Maniphest Tasks: T1191, T6228

Differential Revision: https://secure.phabricator.com/D10623