[Epic] Document master discovery

[patiencedaur]

Dev Q1 ref.: https://github.com/tarantool/roadmap-internal/issues/58
PM ref.: https://www.notion.so/tarantool/e4ec3572492844fca636db6aa6fe83e8

SME: @ sergepetrenko

Tasks

• [5pt] VShard router master discovery #2268 5pt
• [3pt] Document the new pub/sub system and its events #2551 3pt

Details

Currently, the user has to write custom code for node polling (box.info election state) so that a Tarantool cluster can discover the leader (master).

The dev team is developing a connector to the whole replicaset that would discover the master node without the user's involvement. (All the Tarantool connectors in many languages communicate with a single node, not with the whole replicaset.)

[patiencedaur]

Net.replicaset has not been implemented yet and won't be included in the 2.10. release. Removing it from the epic.