nghttpx: Fix heap-use-after-free in ClientHandler object

This bug was found by scan-build
nghttpx: Shut up scan-build
Upate to draft-15

* Add NGHTTP2_HTTP_1_1_REQUIRED error code
* Allow transmission of WINDOW_UPDATE on reserved (remote)
* Allow reception of WINDOW_UPDATE on reserved (local)
* Treat frame larger than MAX_FRAME_SIZE as FRAME_SIZE_ERROR

ALPN identifier is still h2-14 to continue interop, since draft-14 and
-15 are binary compatible.  The new error code was added in draft-15,
but HTTP/2 allows extensions can freely add new error code, so it is
not a problem.
Rename max_header_set_size as max_header_list_size
Code cleanup
Remove unused functions