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replication: add changelog and doc for WAL GC of anon replicas

Closes #10755

NO_TEST=no behavior changes

@TarantoolBot document
Title: WAL GC of anonymous replicas
Since: 3.3

Now anonymous replicas retain required xlogs just as regular ones.

Since anonymous replicas can suddenly disappear, a new function
`box.ctl.replica_gc(uuid)` is introduced. It has an only argument,
UUID of replica that must be a string. If the replica is connected right
now or there is no known replica with such UUID, an error is thrown.
The function can be used to cleanup state of a replica that is gone for
a while: it removes WAL GC state of the given replica and deletes it if it
is anonymous. Regular replicas are not evicted from the cluster. By the
way, there is a function `box.info.replication_anon()` that can be used
to list all anonymous replicas so one can deactivate them with the newly
introduced function - we should mention it somewhere around.

Also, anonymous replicas are deactivated automatically if they haven't
been in touch for too long. Expiration timeout can be controlled by the
new `box.cfg.replication_anon_ttl` option, the default value is
3600 seconds (1 hour).

IPROTO protocol updates:
1. IPROTO_FETCH_SNAPSHOT - is populated with optional
   `IPROTO_INSTANCE_UUID` key. If it is passed, sender is considered as
   a replica and its WAL GC state is registered. If the key is not
   passed, behavior is the same as before.
2. IPROTO_SUBSCRIBE - now WAL GC state is unconditionally created for a
   sender, either anonymous or regular.

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