Ballot is a message sent in response on vote request, which is
sent by applier first thing after connection establishment.

It contains basic info about the remote instance such as whether
it is read only, if it is still loading, and more.

The ballot didn't contain a flag whether the instance is
anonymous. That led to a problem, when applier was connected to a
remote instance, was added to struct replicaset inside a struct
replica object, but it was unknown whether it is anonymous. It was
added as not anonymous by default.

If the remote instance was in fact anonymous and sent a subscribe
response back to the first instance with the anon flag = true,
then it looked like the remote instance was not anonymous, and
suddenly became such, without even a reconnect. It could lead to
an assertion.

The bug is hidden behind another bug, because of which the leader
instance on boostrap registers all replicas listed in its
box.cfg.replication, even anonymous ones.

The patch makes the ballot contain the anon flag. Now both relay
and applier send whether their host is anonymous. Relay does it by
sending the ballot, applier sends it in scope of subscribe
request. By the time a replica gets UUID and is added into struct
replicaset, its anon flag is determined.

Also the patch makes anon_count updated on each replica hash table
change. Previously it was only updated when something related to
relay was done. Now anon is updated by applier actions too, and
it is not ok to update the counter on relay-specific actions.

The early registration bug is a subject for a next patch.

Part of #5287

@TarantoolBot document
Title: IPROTO_BALLOT_IS_ANON flag

There is a request type IPROTO_BALLOT, with code 0x29. It has
fields IPROTO_BALLOT_IS_RO (0x01), IPROTO_BALLOT_VCLOCK (0x02),
IPROTO_BALLOT_GC_VCLOCK (0x03), IPROTO_BALLOT_IS_LOADING (0x04).

Now it gets a new field IPROTO_BALLOT_IS_ANON (0x05). The field
is a boolean, and equals to box.cfg.replication_anon of the
sender.