'mysqli' DB_TYPE fixes and setting it as the default DB_TYPE if the mysqli extension is available.

This replaces #1827.

Fixes to the 'mysqli' DB_TYPE. No need to make it transactional since it already is. I was previously mislead by the comment in the 'mysqli' db driver, so I removed that as well.

Also added the "perf" library for the mysqli driver.

This also ensures that the mysqli extension is available before setting the DB_TYPE to 'mysqli'. If it's not available, the fallback will be to the old 'mysqlt' DB_TYPE.