If "utf8mb4" is unavailable on the client, fall back to "binary"

Summary:
Fixes T11291. It appears the issue there is:

• Server which supports utf8mb4.
• Client which does not.
• The client fails to select utf8mb4 and falls back to utf8, which does not support 4-byte characters.
• Inserting 4-byte characters fails.

The fix here is to fall back to binary instead of utf8. From T11291#185523, this appears to fix the issue in an environment where it previously reproduced.

This also appears fine locally (modern client + modern server), if I force a fallback from utf8mb4.

This should be free from weird side-effects (we verify UTF8 when building queries anyway; we use binary columns on older MySQL).

We could also just fail here, but that would probably force a large set of users running old MySQL to upgrade.

Test Plan:

• See discussion in T11291.
• Locally, forcing this fallback causes no obvious problems.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11291

Differential Revision: https://secure.phabricator.com/D16279