Bug description:

Raft has its own instance_id field, which gets set right in initial box.cfg() call, after recovery/join finishes.

For an anonymous replica this field remains 0, as an anonymous replica id is 0. But it isn't updated when an anonymous replica registers. This triggers an assertion in debug build, and leaves Raft in inconsistent state (with wrong instance id) in release.

Steps to reproduce:
Start 2 tarantool instances in interactive mode:

-- Instance 1.
-- Step 1.
box.cfg{
    listen = 3301,
    election_mode = 'candidate',
}
box.schema.user.grant('guest', 'replication')

-- Instance 2.
-- Step 2.
box.cfg{
    replication = 3301,
    replication_anon = true,
    read_only = true,
}

-- Step 3.
box.cfg{replication_anon = false}

In debug build, instance 2 will fail with an assertion:

./src/lib/raft/raft.c:476: raft_leader_see: Assertion `source > 0 && source < VCLOCK_MAX' failed.

In release build, it'll silently ignore the instance id mismatch, leading to something bad later on.