tarantool / tarantool / 12060

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replication: display downstream status at upstream

This fix improves 'box.info.replication' output.
If downstream fails and thus disconnects from upstream, improve
logging by printing 'status: disconnected' and error message on
both sides (master and replica).
Add relay_state { OFF, FOLLOW and STOPPED } to track replica
presence, once connected it either FOLLOW or STOPPED until master
is reset.

New in second version (amended with previous commit):
1. Do not destroy relay on error. Now destroy only with replica.
2. For diagnostic box.info.replication prints last error if STOPPED.

Closes #3365

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