replication: gather all replicaset variables in struct

There is already a handful of global variables describing the replica
set state and there is going to be more so let's consolidate them in
a singleton struct:

  replicaset => replicaset.hash
  replica_pool => replicaset.pool
  anon_replicas => replicaset.anon
  replicaset_vclock => replicaset.vclock

While we are at it, let's also move INSTANCE_UUID definition from
xrow.c to replication.cc, where it truly belongs. The only reason
I see for it to be defined in xrow.c is to compile vinyl unit tests
without linking replication.o, but we can easily circumvent this by
defining INSTANCE_UUID in vy_iterators_helpers.c.

Suggested by @kostja