Allow postgres locks metric to count any type of lock

Replaces an incomplete whitelist of possible lock types with an empty hash having default value of zero. This avoids an error when the [postgres lock type](http://www.postgresql.org/docs/9.2/static/explicit-locking.html#LOCKING-TABLES) is not in the whitelist.

This change should future-proof the metric against changes to existing postgres lock types or addition of new lock types.

One example that would raise exception prior to this change is when the mode column in the pg_locks database table contains "AccessShareLock".