A few fixes to VQ book-keeping, which get the tests passing again:

* Don't count messages / bytes read during read, do so in
  betas_from_index_entries since unacked msgs don't count
* Introduce qi_pending_ack to prevent us trying to page out messages that
  don't touch the store
* Fix purge bookkeeping, account for the fact messages can be paged in to be
  purged