Replace deprecated grpc.WithTimeout() with alternative grpc.DialContext()

[meling]

This line in node_gen.go should be replaced with

	n.conn, err = grpc.DialContext(ctx, n.addr, opts...)

However, this will impact the Manager API since the ctx must be supplied when creating the manager, so that each node gets the same timeout:

	ctx, cancel := context.WithTimeout(context.Background(), time.Second)

Also we need to consider if each node should get its own ctx, and we should probably handle cancel() calls in the Node.Close() or something.

Need to consider if this should be part of the manager api or hidden from the user.

[meling]

Closed by #49