WARNING should be WARN.

Normally stream is in dependency tree, but sometimes it isn't.  For
example, client does not put pushed reserved stream in dependency
tree.

This is not obvious but it makes intermediaries flush and forward DATA
frame boundary without excessive buffering.  Since we have different
TCP connections frontend and backend, this may not work.  This is
still experimental.

Use the same behaviour the current Google server does: start with 1300
TLS record size and after transmitting 1MiB, change record size to
16384.  After 1 second idle time, reset to 1300.  Only applies to
HTTP/2 and SPDY upstream connections.

Now the number of -v option specifies verbosity level.  Current all
verbose output are turned on for at lest one -v option, except for the
debug output for each data chunk which is only turned on more than one
-v options.

This change fixes the bug that stream is out of dependency tree if the
number of nodes in a dependency tree which we add new node to is
already maximum (NGHTTP2_MAX_DEP_TREE_LENGTH) and the number of
maximum concurrent streams is more than more than
NGHTTP2_MAX_DEP_TREE_LENGTH.

* 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.