replication: merge replica_by_id into replicaset

For some reason the replica_by_id member (which is an
array of pointers) is allocated dynamically. Moreover
VCLOCK_MAX = 32 by now and extending it to some new
limit will require a way more efforts than just increase
the number.

Thus reserve memory for replica_by_id inside replicaset
statically. This allows to simplify code a bit and
drop calloc/free calls.

The former code comes from edd76a2a without any
explanation why the dynamic member is needed.

Acked-by: Konstantin Osipov <kostja.osipov@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>