CouchElephant 1.3.0

A cleanup review of sync.py and passes.py found the cause under four fixes
that shipped this week, two silences, and two bugs nobody had reached yet.

Fixed

• A broadcast's id is now minted from the broadcast. It used to prefer
  Plex's own Media.id, which is the one value in the payload that moves: a
  guide refresh renumbers airings without changing anything about them, and one
  game here collected nine ids over three weeks. Every lookup keyed on the id
  had to be taught to resolve a retired one, and four of them were taught one at
  a time, each after failing in front of somebody.

  An id is now the programme, the channel and the start time. A renumber
  produces the same id. A re-time still changes it, which is correct: that is a
  different broadcast and the app has to notice. Bookings made before this are
  re-keyed when the app starts.

  Two more places had the same fault and nobody had reached them. The guide's
  "Being recorded" filter stopped matching a booked game after a refresh, and
  cancelling a recording by hand answered "CouchElephant did not schedule this"
  for a recording CouchElephant had scheduled.

• A re-point that Plex refused was counted and never mentioned. It
  incremented the failure count without adding to the list the notices are
  built from, so nothing was raised. That is the silence this whole check
  exists to remove.

• A booking could be trapped in a permanent failure. Plex was asked about
  the subscription before our own guide was consulted, so a booking whose
  subscription Plex had lost and whose broadcast had left the guide went to the
  re-book path. There is nothing to re-book from, so it failed and raised a
  notice on every sync, for ever, while the branch that exists to cancel
  exactly that booking was unreachable for it.

• A repaired booking's history row could name two broadcasts at once,
  recording the old airing id beside the new channel and start time.

Changed

• One re-book path instead of two. They had already disagreed about when to
  forget the old booking and whether to write down a failure.
• Guide and schedule pulls share one prune helper rather than two copies.
• The booking check counts in one place, so a new outcome cannot be counted in
  one half and forgotten in the other.

Performance

Measured against the live install: about 16,700 programmes and 22,800 airings
per sync, hourly.

• A channel is written once per sync instead of once per broadcast: about
  22,700 redundant writes and the same number of redundant title parses, gone.
• Two indexes on plex_grabs, which holds the whole DVR schedule and was
  scanned in full by three hot questions, one of them once per airing of a
  programme.
• Asking whether a team pass matches anything now stops at the first match
  instead of hydrating every match in the window.
• Re-pointing a booking asks for one programme in SQL rather than the pass's
  whole thirty-day window, and reads the pass rows once per check rather than
  once per booking.
• A pass run reads the last sync time once rather than once per game.