Add support for mysqli extension

DatabaseBase::factory()'s parameter array now takes an optional
'driver' variable to allow specifying a specific driver for the
database class if we support more than one.

This is so we can gracefully support mysql/mysqli as the former is
deprecated, but the concept could be extended to other databases
as well.

Bug: 45288
Co-Author: Chad Horohoe <chadh@wikimedia.org>
Change-Id: I6733fe21c4aa7443e409c5dfa7c789552b2b62b7