Add query methods to Store trait

[sgwilym]

Adds query trait methods as detailed in #21 (comment):

• query
  • query entry by pair of path and subspace id
    • yields Entry, AuthorisationToken, available payload prefix (length)
    • option to filter out results with incomplete payloads
  • query entries by area (of interest)
    • all of the functionality of singleton queries, also
    • ordering results (arbitrary, PTS (optionally reversed), TSP (optionally reversed), SPT (optionally reversed)) (?)
  • subscriptions: all queries should also work as long-lived subscriptions
    • this includes subscription to payload prefix appending
    • this also includes notifications about overwriting and forgetting things
  • persistent subscriptions
    • all subscription notifications come with a u64 progress id, client code can stop consuming a subscription at any time and can later (even between program shutdown and restart) resume it at the same point by supplying its progress id
    • this is a best-effort service, the storage may reject a resumption because it is too outdated
      • a simplistic implementation can effectively opt out of providing persistent subscriptions by simply considering all ids as outdated
    • forgetting entries/payloads can cause progress ids to become outdated, unless the storage tracks the things it forgot
      • this is why all forget functionality comes with a flag whether it should be completely traceless or whether the storage is allowed to track the act of forgetting

To be merged into #21

[AljoschaMeyer]

Lgtm now