replication: omit 0-th vclock component in replication responses

If an anonymous replica is promoted to a normal one and becomes
replication master later, its vclock contains a non-empty zero
component, tracking local changes on this replica from the time when it
had been anonymous. No need to pollute joining instance's vclock with
our non-empty 0 component.
When an anonymous replica reports its status to a remote instance it
should also hide its 0-th vclock component.

This is needed for backward compatibility with old instances, which
don't ignore 0th vclock component coming from a remote instance by
default.

In order to do so, introduce a new function - vclock_size_ignore0(),
which doesn't count 0th clock component, and patch xrow_encode_vclock()
to skip 0th clock component if it's present.

Also make sure that new instances ignore 0th vclock component coming
from an unpatched remote instance.

Follow-up #3186
Prerequisite #4114