lnd: add new "perm" bit to the ConnectPeer RPC call

This commit modifies the ConnectPeer RPC call and partitions the
behavior of the call into two scenarios: the connection should be
persistent which causes the call to be non-blocking, and the connection
should only attempt to connect once — which causes the call to be
blocking and report any error back to the caller.

As a result, the pendingConnRequest map and the logic around it is no
longer needed.

rpcserver.go

@@ -215,15 +215,13 @@ func (r *rpcServer) ConnectPeer(ctx context.Context,
 		ChainNet:    activeNetParams.Net,
 	}
 
-	peerID, err := r.server.ConnectToPeer(peerAddr)
-	if err != nil {
+	if err := r.server.ConnectToPeer(peerAddr, in.Perm); err != nil {
 		rpcsLog.Errorf("(connectpeer): error connecting to peer: %v", err)
 		return nil, err
 	}
 
-	// TODO(roasbeef): add pubkey return
 	rpcsLog.Debugf("Connected to peer: %v", peerAddr.String())
-	return &lnrpc.ConnectPeerResponse{peerID}, nil
+	return &lnrpc.ConnectPeerResponse{}, nil
 }
 
 // OpenChannel attempts to open a singly funded channel specified in the