fix: expel is blocked by master switchover even if replicas are offline

Summary

• fix: don't update current_master_id until it synchronizes

This patches changes when we update current_master_id during consistent master switchover. Now it is updated as soon as we determine that the new master has synchronized it's vclock with the old master.

As a result we can now block expel of a non-last replica in cases where other replicas are offline, which is crucial because instances can easily become offline due to temporary network issues.

• fix: expel is blocked my master switchover even if replicas are offline

• fix: change definition of Instance::may_respond

Now we assume instance may not respond if

• target state is Offline (non-graceful assumed)
• current state is Expelled

• test: add a failing test for bucket rebalancing before instance expel

• test: don't panic libtestplug.so in case of unauthorized RPC

• refactor: cleanup code in resolve_rpc_target

• Cherry-pick to: none
• Closes: #186 (closed), #1055 (closed)
• Docs follow-up: docs#395 (closed)
  • changed return value of proc_replication
  • new RPC proc_replication_sync