Add a setup issue about small "max_connections" settings

Summary:
Fixes T11683. Likely as a result of the persitent connections change, more users are seeing MySQL connection limit errors.

The persistent connections change means we use fewer connections at the high end, but I'm guessing PHP is keeping some more connections around in the pool, so while high-traffic hosts use fewer connections, low-traffic hosts now use more.

Raise an explicit setup warning about this. Users should be adjusting it anyway, there's no value to leaving it at extremely low default and connections are baiscally free until you run out of outbound ports.

Test Plan: {F1844630}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11683

Differential Revision: https://secure.phabricator.com/D16586