code: boolean setReplica( H2ONode h2o ) {
      assert _key.home(); // Only the HOME node for a key tracks replicas
      assert h2o != H2O.SELF;     // Do not track self as a replica
      if( !read_lock() ) return false; // Write-locked; no new replications.  Read fails to read *this* value
      // Narrow non-race here.  Here is a time window where the rwlock count went
      // up, but the replica list does not account for the new replica.  However,
      // the rwlock cannot go down until an ACKACK is received, and the ACK
      // (hence ACKACK) doesn't go out until after this function returns.
      replicas()[h2o._unique_idx] = 1;
      // Both rwlock taken, and replica count is up now.
      return true;
    }
docs: Atomically insert h2o into the replica list; reports false if the Value
  flagged against future replication with a -1.  Also bumps the active
  Get count, which remains until the Get completes (we receive an ACKACK).