improve data migration performance
possible alternative: use a global MVar for whether a migration is in progress (finer grained might be possible as well), thus serialising the migrations.
problem: possible message loss when not all subscriptions are transferred yet and new incoming messages are only forwarded to already migrated subscriptions, but won't reach yet-to-be-migrated ones.
counter measure: stop semantics, during migration new messages are rejected and need to be queued for retry at the senders.
deal with sending DHT messages to own node
Implement or mock transfer of subscription data when joining/ leaving/ rebalancing