Improvement for postgresql plugins

[kuriyama]

(1) postgres_users plugin returns different queries between config and fetch request. I think the same condition should be used in both of config/fetch.

(2) postgres_locks_ returns full list of lock types when config request, but it only returns having-value locks to fetch request. Attached patch is not clean/simple, but it works to return 0 values to shut up warnings in node.log.