neutrino: never write filters to disk unless signaled, instead utilize LRU cache

[Roasbeef]

neutrino: never write filters to disk unless signaled, instead utilize LRU cache

[rawtxapp]

This looks like a relatively good first issue, I'd like to take a stab at it if it isn't assigned to anyone, what do you think @Roasbeef ? If yes,

I looked through the code and instead of putting the caching inside GetCfilter, what do you think of putting it directly inside the FilterStore ? It seems like a logical place and that way GetCfilter logic would stay the same (other than just passing an extra persistToDisk bool to PutFilter).

For the LRU, what do you think of using an existing lib like golang-lru ? It also has Adaptive Replacement Cache and TwoQueueCache implementations that we could experiment with.

[Roasbeef]

Ideally we don't bring in additional lib just to implement an LRU cache. Also this should be done outside of the FilterStore all together. This is a caching layer that sits on top of the filter store.