Move some stuff from DatabaseBase to DatabaseMysql

Move some stuff from DatabaseBase to DatabaseMysql

I moved a number of methods and marked them abstract in DatabaseBase,
namely those that met the following criteria:

1) Used some mysql_* function, so would obviously fail on any other DB.
2) Were already implemented by all five non-MySQL subclasses.

It would be reasonable to revisit these and make the less essential ones
return a dummy value instead of being abstract.  And document them.  And
move more methods that are MySQL-specific.  But one thing at a time.

I also split the classes Database, DatabaseMysql, and MySQLMasterPos to
DatabaseMysql.php.

This should theoretically have no practical changes, but it's a bunch of
changed code, so it will possibly break something by accident.

Followup to r51795.