Allow postgres locks to be any possible lock

Removing the white list of potential lock modes in favor of setting a default hash value of zero avoids an error `undefined method `+' for nil:NilClass` in the case that postgres' lock mode was not one of the listed possibilities.

Specifically, the presence of `AccessShareLock` as a mode of postgres' pg_locks table was causing an exception.
Renaming variables
Merge pull request #792 from PaulOstazeski/master

Allow postgres locks metric to count any type of lock