Add RbsrStore trait

[sgwilym]

As described by #21 (comment):

• internal (for replication)
  • all of the public-facing functionality may also be used internally
  • query entries by 3dRange
    • yields Entry, AuthorisationToken, available payload prefix (length)
    • option to filter out results with incomplete payloads (?)
    • optionally order according to the ReconciliationAnnounceEntries:will_sort field
    • optionally as a subscription (?)
  • summarise 3dRange (count, fingerprint)
  • convert area of interest to 3dRange

to be merged into #48

[sgwilym]

note to self: this trait should be moved into a new WGPS crate.