Release v19.100.1

Fixed

• An observer's stored position no longer moves backwards when it changes state while a batch is still being handled, so a later catch-up no longer delivers those events again or counts them twice (#5078)
• Observers no longer keep partitions marked as in flight forever after an upgrade: a marker left by an earlier version is cleared once its partition has been handed to catch-up, replay or failed-partition recovery, and only stays when that recovery could not start (#5078)