Add priority message queue implementation

[peterargue]

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This is a generic implementation of a priority message queue, which utilizes the container/heap implementation.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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[peterargue]

I wish we had golang/go#47632.

How come we are requiring that the PriorityQueue is a heap, instead of the PriorityQueue using a heap and just exposing its own methods that don't use any?

@janezpodhostnik PriorityQueue is a copy of the container/heap example, though I actually sourced it from our network implementation.

It basically is a max heap implementation, just named priority queue since that's the usecase.