Add howto postgresql and how to get locked queries sample

Source: http://big-elephants.com/2013-09/exploring-query-locks-in-postgres/

This MR adds how to get the queries that are waiting on a lock, and how to get a sample for what query are they waiting on.

I'm also adding these scripts to the db4 root folder for brevity

cc/ @stanhu @ahanselka @maratkalibek @northrup @jnijhof @yorickpeterse @MrChrisW