Use more sophisticated cache management in flycheck-ycmd

[abingham]

Right now we clear the flycheck-ycmd cache whenever new parse results come in. It might be better to do more selective cache management, including some LRU cleanup. Or maybe not...the current implementation may be just fine. We need to think about this and choose the best solution.

[abingham]

A possible cache management approach is:

1. Find all affected filepaths, and clear them from the cache
2. For each result, insert into cache
3. Remove things that haven't been used in a while.