Short-circuit the initial catchup message from the left, so that we are sure we receive it before we consider ourselves joined.
Abstract the initial catchup into its own function. Make sure we call check_neighbours beforehand, since that sets left and right.
Merge in bug24929
Merge bug24929.
Merge bug24929
Merge in stable
Remove some unreachable code - members_state will be filled once we have joined, and now we join immediately at startup.
Merge bug26171