Add `ChainMonitor::archive_fully_resolved_monitor_channels`

  Archives fully resolved channel monitors by adding them to a backup
  location and removing them from the primary storage & the monitor set.

  This is useful for pruning fully resolved monitors from the monitor
  set and primary storage so they are not reloaded on every new new
  block connection.

  We also add a new function, `archive_persisted_channel` to the
  `Persist` trait that writes the monitor to an archive storage and
  removes it from the primary storage.