Replace LruCache with an external library

[belak]

I looked at using the python stdlib functools.lru_cache decorator as well, but it doesn't lend itself very well to this use case because there are multiple ways of getting something into the cache.

This drops some code and replaces it with the external library cachetools because it doesn't make much sense to copy an LRU cache in when we could just use a pre-existing one.

[jodal]

👍

Easy choice as cachetools is maintained by @tkem, an active Mopidy contributor, and it is already used by other Mopidy extensions.