As predicted, I ended up unifying the Headers and Push Promise packing.

This is in preparation for reusing the writeResHeaders functionality
of sending continuation frames if the headers are too big.

The API uses loopback to the top handler in order to make sure that
the resources being pushed are related to the resource being fetched.

If the header parameter is nil, we copy the headers from the initiating
request. This is mostly a shortcut for the common case where we don't
want to specify any new request headers.

…client concurrent streams to 100 and make some code refactoring from DanielMorsing.

* add default priority and dependency for push stream;
* add check in processData function for push stream.