Replace use of a PriorityQueue with a list

Description

This replaces the PriorityQueue with a regular Python list, using locks when modifications need to be made to the list to reduce the likelihood of things getting broken by multiple threads interacting with the list. When a list was used previously, all interaction with the list was done once a lock was obtained, but that isn't feasible whilst allowing bots to long-poll.

This merge request, when merged, will address issue/bug: #198 (closed)