Enable master-master replication for better remote hot standby

Today, in Tarantool, you can dynamically change replication sources, or turn replication off. This allows system administrators to simplify upgrade, which is done by moving the write load to replica and propagating replica to master, as well as implement high availability, by detecting when a master goes down, and, again, replacing it with the replica.

But it is still too much labor, and the same can be achieved more easily.

Implement master-master replication, when each instance can both, be a replication source and a destination.
In this mode, whatever changes are made on one instance, get propagated to the other, and vice versa.
When an instance goes down for some reason, it's enough to move all load to the replica, which can from now on accept all updates.
The real magic happens when the dead instance comes back up -- it will automatically connect to the other master and receive all the updates that it missed.