Fix __toString method of DatabaseMysqli

The superclass was casting mConn to string. For Mysql class this
is just the resource number. However for mysqli objects, that is
a fatal. thread_id seemed like the most convinenent id-ish number
to return instead.

Only really noticeable if you have $wgDebugTransactions = true;

Change-Id: I014bb7ab81d18c5bd07a267939b66a0a6161eb8d