nghttp2_on_begin_frame_callback will be invoked when a frame header is
received.

It is not used by library for a while.  It could be used to pass
unsupported extension frames to application, but its interface
requires library to buffer entire frame, which we'd like to avoid.
For unsupported extension frames, we will add new callbacks which does
not require buffering if they are required.

h2-14 now allows extensions to define new error codes.  To allow
application callback to access such error codes, we uses uint32_t as
error_code type for structs and function parameters.  Previously we
treated unknown error code as INTERNAL_ERROR, but this change removes
this and unknown error code is passed to application callback as is.

To make it possible to add new callbacks without bumping so name, we
decided to hide details of nghttp2_session_callbacks.  We provide
setter like functions to set individual callback function.

Merge branch 'window_size_control' of https://github.com/akamai/nghttp2 into akamai-window_size_control

Motivation:

The send window size is currently fixed by a macro at compile time.
In order for users of the library to impact the send window size they
would have to change a macro at compile time. The window size may be dynamic
depending on the environment and deployment scheme. The library users
currently have no way to change this parameter.

Modifications:

Add a new optional callback method which is called before data is sent to
obtain the desired send window size. The callback return value will be
subject to a range check for the current session, stream, and settings
limits defined by flow control.

Result:
Library users have control over their send sizes.

Previously returning NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE from
on_header_callback moves input offset badly and it causes header
decompression error on the subsequent frames.  This commit fix this
bug.

Previously we only update consumed flow control window when number of
bytes read in nghttp2 and spdylay callback is 0.  Now we notify
nghttp2 library the consumed bytes even if number of bytes read > 0.
This change also uses newly added spdylay_session_consume() API, so we
require spdylay >= 1.3.0.

This is partial revert of bbe4f5a3.
Only documentation is reverted.  Since we have 2 queues to handle
maximum concurrent streams, we are not ready to allow immediate frame
submission for pending new frames.

Android lacks /dev/stderr, so directly use /proc/self/fd/2 as default
errorlog-file.  Android does not like O_APPEND for /proc/self/fd/1 and
/proc/self/fd/2, so omit the flag for these paths.

This commit makes handling of outgoing HEADERS and PUSH_PROMISE in the
same priority of other frames on the stream, so these frames are
processed in the order they are submitted.  This allows application to
submit frames to a stream returned by nghttp2_submit_{request,
headers, push_promise} immediately.  The only exception is
WINDOW_UPDATA frame, which requires nghttp2_stream object, which is
not created yet.