Move MySQL-specific fakeMaster/fakeSlave stuff to DatabaseMysqlBase

Probably the fakeMaster/fakeSlave stuff was originally intended to be
generic, but the existing code in Database.php is making some very
MySQL-specific assumptions. So let's evict it from Database.php (except
for the minimal support functions) and put it in DatabaseMysqlBase where
it makes more sense.

This also takes care of fixing the breakage described in I5d2a1696 by
reverting Id6268193 and the problematic bit of I364e192e (again).

Change-Id: I3dc6ca216bf2c2f07d3090e86f2539292e1fa86b