replication: fix replica disconnect upon reconfiguration

Replication reconfiguration used to work as follows: upon receiving a
new config disconnect from all the existing masters and try to connect
to all the masters from the new config.

This lead to instances doing extra work when old config and new config
had the same nodes in them: instead of doing nothing, tarantool
reinitialized the connection.

There was another problem: when an existing connection is broken, master
takes some time to notice that. So, when replica resets the connection,
it may try to reconnect faster than the master is able to notice its
absence. In this case replica wouldn't be able to reconnect due to
`duplicate connection with the same replica UUID`. So replication would
stop for a replication_timeout, which may be quite large (seconds or
tens of seconds).

Let's prevent tarantool from reconnecting to the same instance, if there
already is a working connection.

Closes #4669