proposal: convert searcher into an interface to allow database storage

[adamdecaf]

Running multiple instances of Watchman can run into state issues where one restarts/refreshes data out of sync from other instances. Instead we should open up the possibility for Watchman to store (and search) list data inside a database.

It's probably safe to say a NoSQL datastore makes more sense as the query fields across lists can vary and their format isn't always stable. Setting up SQL tables for each list feels over-engineered to me.