WIP, possibly broken (although tests ok)

New ORM::Db init for multiple $dbh handles (for read/write distributing):

ORM::Db -> init( { read => [ $dbh1, $dbh2, $dbh3 ... ],
		   write => [ $dbh4, $dbh5 ... ] } );

Model::Class -> set_dbh( { read => [ $dbh1, $dbh2, $dbh3 ... ],
			   write => [ $dbh4, $dbh5 ... ] } );

Less usable:

Model::Class -> set_read_dbh( [ $dbh1, $dbh2, $dbh3 ... ] );

Model::Class -> set_write_dbh( [ $dbh4, $dbh5 ... ] );