config: anonymous replica is now supported

[TarantoolBot]

Related dev. issue(s): tarantool/tarantool#9432

Product: Tarantool
Since: 3.0
Root document:

• https://www.tarantool.io/en/doc/latest/reference/configuration/configuration_reference/#configuration-reference-replication-anon
• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_info/replication_anon/
• https://www.tarantool.io/en/doc/latest/dev_guide/internals/iproto/replication/#general (and other <cfg_replication-replication_anon> links)

SME: @ Totktonada

Details

replication.anon: true option is now working.

There are configuration constraints that are related to anonymous replicas.

• A replicaset must contain at least one non-anonymous instance.
• An anonymous replica can't be configured as writable instance using database.mode or <replicaset>.leader options.
• An anonymous replica can't be configured with replication.election_mode equals to candidate, voter or manual (only off is allowed).

A few more nuances about anonymous replicas:

• Anonymous replicas are filtered out from default upstream list.
• A replication.failover: election replicaset can contain anonymous replicas, but replication.election_mode defaults to off for them (unlike non-anonymous instances, where the default is candidate).
• replication.failover: supervised skips anonymous replicas, when choosing a bootstrap leader.
• A anonymous replica can joined a replicaset, which has all the instances in read-only mode (unlike a non-anonymous instance).

See details in tarantool/tarantool#9432 and tarantool/tarantool#9418.

Requested by @ Totktonada in tarantool/tarantool@5ba3ace.