[System] WebConnection and WebConnectionStream code cleanups.
* Merged WebConnection.EndWrite() and EndWrite2().
* WebConnectionStream.BeginWrite(): Use the same async-write
callback for both chunked and non-chunked; remove the single-use
small WriteRequestAsync() and inline it here to make the code
more readable.
* WebConnectionStream.WriteRequestAsyncCB(): Now called for all
async writes, call WebConnection.EndWrite() here.
The code is now more readable and easier to understand. This is
the first part of a series of cleanups, which should eventually
lead towards full async and 100-continue.