Fix FIFO usage

Authored by elvisangelaccio on May 31 2020, 9:43 PM.

Description

Fix FIFO usage

m_propriorityQueue and m_queue are used as FIFO queues, so we need
to use takeFirst() instead of takeLast() when removing elements
from the queue.

While at it, add a comment so that we will remember these are FIFO
queues.