Added a method to cap the size of the cache in DijkstraShortestPaths and DijkstraDistance

[lhepto]

This is a proof of concept to show the benefit of implementing a cap on the size of the cache. It allows the user to perform a large number of shortest path calculations on large graph, continuously without GC or memory issues, when they are memory constrained in the VM. See the new test cases TestCachingLargeGraph for an example. Tested with VM options: -Xms128m-Xmx750m

[tomnelson]

Was there ever a plan to merge this?

[jrtom]

It's on my list of things to look at before the 3.0 release. Haven't gotten to it yet.