Move server's WINDOW_UPDATE sending logic into the serve loop.

Previously the http.Handler (via requestBody) were just blinding
writing telling the serve loop to send the WINDOW_UPDATE frames, but
the serve loop didn't really know what was going on.

Now the requestBody.Read instead tells the serve loop that n bytes
were read on a given stream, so the server can be smarter.

This opens the door to coalescing updates, and suppressing pointless
ones (like the new TODO notes: no need to send a stream-specific
WINDOW_UPDATE when the peer has closed their side already)

No (intentional) changes in behavior in this change, though.
Updated TODO
New TODO, related to a known crash
Fix copy/paste typo in server_test.go