Add a setup warning about innodb_buffer_pool_size

Summary: Fixes T6119. This is a little fuzzy, but generally bumping up innodb_buffer_pool_size to something bigger than the default (which is often anemic, at 8M) is desriable, and it seems like it will fix the specific issue a user encountered in T6119.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6119

Differential Revision: https://secure.phabricator.com/D10630