* do not allow "deleting" id of self from _cluster table
* assume struct replica for self is always present, even
  when replication is off
* update cluster.test.py, remove tests for removal of self
  instance id
* use struct replica self->id instead of global instance id
  wherever necessary
* read only mode is now defined by is_ro alone, since
  the local instance always has an assigned id

This cherry-picks some changes from branch remove-tx-vclock,
and follows up on the removal of recovery->replica_id member.