Fixes #3377: Cascade power draw calculation on all downstream power ports

[hSaria]

Fixes: #3377

During the calculation of power draw, include all downstream power ports (i.e. non-immediate). To maintain efficiency, each power outlet caches the upstream and downstream power ports.

A power topology change will update the cached fields. The trigger for a topology change is the cable being updated/deleted (in dcim.signals).

downstream_powerports flattens what would otherwise be a recursive process during power calculation to a single query.

upstream_powerports is used as part of the power calculation to update the cached fields for any power outlet affected by the topology change.

This PR is a refactor following the extended discussion in #3916, which has more information on this topic.

[jeremystretch]

Sorry this has stagnated for so long. I took a look at it, but honestly it just introduces too much logic and overhead to support a single fairly niche feature. The implementation of two new ManyToManyFields is especially concerning: We avoid M2M fields wherever possible as a rule due to the difficulties they impose on forming efficient queries. The denormalization of power outlet relationships on top of that leads me to suspect this will be a frequent source of bugs that I'm just not willing to accept at this point in time.

Given that this has been open for a few months and none of the other maintainers has expressed interest in it, I'm going to close it out. However, I suspect the overall approach conveyed in the PR is still viable to some degree, and it will serve as a reference for the eventual implementation of #3377 once it gets roadmapped. Thank you for your hard work on this, and I'm really sorry we can't move forward with it at this time.