Add database index to workbench_path_revision table

We also need an index on alias. This is important for large sets of data.

mysql> explain  SELECT workbench_path_revision.nid FROM workbench_path_revision JOIN node ON workbench_path_revision.nid = node.nid AND workbench_path_revision.vid = node.vid WHERE workbench_path_revision.alias = 'some.html';
+----+-------------+-------------------------+-------+-----------------+---------+---------+-----------------------------------------------+--------+-------------+
| id | select_type | table                   | type  | possible_keys   | key     | key_len | ref                                           | rows   | Extra       |
+----+-------------+-------------------------+-------+-----------------+---------+---------+-----------------------------------------------+--------+-------------+
|  1 | SIMPLE      | node                    | index | PRIMARY,vid     | vid     | 5       | NULL                                          | 574224 | Using index |
|  1 | SIMPLE      | workbench_path_revision | ref   | PRIMARY,nid,vid | PRIMARY | 8       | some.node.nid,some.node.vid |      1 | Using where |
+----+-------------+-------------------------+-------+-----------------+---------+---------+-----------------------------------------------+--------+-------------+

mysql> alter table workbench_path_revision add index (alias);

mysql> explain SELECT workbench_path_revision.nid FROM workbench_path_revision WHERE workbench_path_revision.alias = 'data.html';
+----+-------------+-------------------------+------+---------------+-------+---------+-------+------+--------------------------+
| id | select_type | table                   | type | possible_keys | key   | key_len | ref   | rows | Extra                    |
+----+-------------+-------------------------+------+---------------+-------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | workbench_path_revision | ref  | alias         | alias | 767     | const |    1 | Using where; Using index |
+----+-------------+-------------------------+------+---------------+-------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)