Switch PID registry to associative array

[vinjana]

Currently the PID registry is a plain array. With associative arrays one can give a names to the PIDs and report them upon errors, which is great for debugging and development.

Make the PID registry an associative array. See here for extensive good docu: https://www.artificialworlds.net/blog/2012/10/17/bash-associative-array-examples/.