re-write

[ahdinosaur]

this is a tracking pull request for re-writing fs-db as fsdown, to be compatible with abstract-leveldown, which provides us with the levelup interface.

TODO:

• works somewhat (is useful in another task)
• works well (abstract tests pass)
• fs tests pass

implementing it with only read and write streams is not the best idea, we should move to something that parses the files into lines with a known start and end point, so from there we can do small atomic reads and writes, but eh i'm going to sit on it for a bit till a plan comes along. (maybe involving linez and extending memdown like jsondown does)

[ahdinosaur]

turns out implementing a leveldown interface with abstract-stream-leveldown isn't as easy as i had hoped. to push holodex/holodex#82 along, i'm going to throw out being leveldown compatible and just export a constructor with createReadStream and createWriteStream where the writes flush the entire file. punting on the long-term plan for now, deferring that to holodex/holodex#80.