Incorporate lru-cache library for basis of async cache logic

[cmdcolin]

This is a proposal to swap out the core async cache logic for lru-cache

lru-cache is a battle-tested library and would be beneficial to incorporate as the basis for our system, as it is hard to reason about the exact nature of async caching

This PR swaps out our logic for lru-cache by making "class AbortablePromiseCache extend LRUCache" (the class from lru-cache's library)

The hope would be to achieve less memory leaks and control memory usage better in jbrowse.

Because we extend the LRUCache class, this does change the API interface. "Wrapping" the class and exposing our own interface could allow us to keep our old interface

Note: I'm only acting on a hunch, not out of a specific memory leak that I observed....

[cmdcolin]

one important note is that we retain almost full test compatibility with current master, and any test changes i made seemed to be essentially unneeded concepts to preserve