path caching

[kostmo]

Closes #1569

Performance

Path cache invalidation upon world modifications (i.e. entities inserted or removed) entails iterating over all of the previously-cached paths here. For efficiency's sake, we avoid iterating over "all existing robots".

Any scenario that does not use the path command is entirely unaffected by this change.

Demo

Previously, this demo was virtually unplayable, since when moving between widely-spaced "clusters" of flowers, an expensive A-star search was invoked at almost every tick. Now, the vast majority of moves utilize the cache, and the demo exhibits minimal stuttering (e.g. when a single A-star search is performed when moving between distant clusters).

scripts/play.sh -i scenarios/Fun/horton.yaml --autoplay --speed 7

Event log

An event log specific to the path cache is maintained with its own ring buffer:

scripts/play.sh -i scenarios/Testing/1569-pathfinding-cache/1569-harvest-batch.yaml --autoplay

and view http://localhost:5357/paths/log

[kostmo]

Cache invalidation callback is here.

[byorgey]

@kostmo , thanks so much for the detailed comments and the amazing attention to detail with the whole cache logging system + tests!

I must admit I was skeptical of the idea of shortest path caching initially --- I was worried about performance, etc. --- but (1) the Horton's flower scenario is fun, and is a convincing demonstration of the utility of path caching, and (2) I am convinced that this will have minimal/zero impact whenever the path command is not being used.

I would love to eventually see a series of challenges introducing the path command (not saying you have to develop them).