single LRU cache utilitiy to replace both implementations

[oribarilan]

Currently, we have 2 cases where we implement almost identical LRU caches:

1. the lens explorer, caching functions/methods from the lsp
2. the blame cache, caching blame information

creating a single utilitiy file for LRU cache can clean this up, and also introduce a more efficient (and better tested) LRU mechanism.