Abstract out mysql_field_type() call from DatabaseBase

DatabaseMysqlBase::fieldType() is used in the maintenance script
convertLinks.php for upgrading from MediaWiki 1.2 or earlier. This
script was failing when the mysqli extension was enabled.

Follows-up b1cc3511fd7a, b2d64432ee02, and 00299ecb9515.

Change-Id: I5d888f4dd344ecba15506023d35f6d2d6e24cfd8