Change the QHttpServerResponder API to support the common functionality of HTTP 1.1 and HTTP 2

The API of the QHTTPServerResponder class is tailored for HTTP 1.1. Change it to make it possible to use for both HTTP 1.1 and HTTP 2.

This means we must remove some of the low-level functions from the API and add some new functions:

• We can no longer support writing the status separate from writing headers because :status is a pseudo-header to be sent with the other headers in HTTP 2.
• We can also no longer write to the body without knowing if there will be more data left to write because HTTP 2 uses flags in the frames sent to determine when there is no more data, and when the socket is closed from the sender.

The API should look like this:

• Keep the write functions that allows the user to write a single response back with different data types. Also keep the sendResponse function to support sending a QHttpServerResponse back.
• Remove the writeStatusLine, writeHeaders and writeBody functions.
• Add a three sets of functions to let the user begin a chunked write, write a single chunk of data, and finalize the writing of chunked data. Chunking is HTTP 1.1 terminology, but it is the same as sending multiple DATA frames in HTTP 2. Other naming schemes are possible.