(release/v20.03) Update nextRaftId when starting a node with a raftId…

… > 0. (#6598)

If we don't do that, the next node that starts with an ID of zero
(auto-assign) will be assigned an existing ID.

DGRAPH-2496

(cherry picked from commit a4f21f4)

dgraph/cmd/zero/zero.go

@@ -502,6 +502,9 @@ func (s *Server) Connect(ctx context.Context,
 			m.Id = s.nextRaftId
 			s.nextRaftId += 1
 			proposal.MaxRaftId = m.Id
+		} else if m.Id >= s.nextRaftId {
+			s.nextRaftId = m.Id + 1
+			proposal.MaxRaftId = m.Id
 		}
 
 		// We don't have this member. So, let's see if it has preference for a group.