Enable ServeHeader again.

[peilun-conflux]

This can help to avoid some unwanted messages.

And when running python tests, this can prevent a node to request blocks from out python mini node, making tests less prone to unexpected timeout just because of unluck.

[peilun-conflux]

The current implementation of broadcasting ServeHeader at phase change is not working.

We should probably exchange this in handshaking or do it periodically.