Previously we empties request headers after they are sent to
downstream in order to free memory.  But it turns out that we use
request headers when rewriting location header response field.  Also
user reported that request headers are useful to add new features.
This commits defers the deletion of request headers to the point when
response headers are deleted (which is after response headers are sent
to upstream client).

Even after on_stream_close_callback, Http2DownstreamConnection is
still alive and upstream keeps sending response to the client.  The
consumed bytes are processed normally (data_source_read_callback) and
also we have a code to consume all allocated bytes for
Http2DownstreamConnection object when it is deleted.  This means that
we don't need to and should not consume response data in downstream
on_stream_close_callback.  If we do, we may get assertion error in
Http2DownstreamConnection::resume_read().

This commit adds test to submit DATA frame twice and fixes the bug
that 2nd DATA is not sent.

Added test for this crash.

This commit moves frame_type parameter of
nghttp2_data_soruce_read_length_callback in front of stream_id
parameter.  The motivation is that other callback is generally put
frame related parameters first.  To make it consistent, we move
frame_type, which is frame ralted parameter, to the left.

Previously we always call on_frame_send_callback before calling
nghttp2_stream_detach_data() after sending DATA frame.  As a result,
even if DATA frame has END_STREAM, application cannot call
nghttp2_submit_data() in on_frame_send_callback because previous data
is still attached.  This commit makes a change so that
nghttp2_stream_detach_data() is called before on_frame_send_callback
so that application can issue nghttp2_submit_data() in the callback.

Now it returns only stream's available remote window size, without
considering connection level window size.  For connection-level window
size, nghttp2_session_get_remote_window_size() is added by this
commit.  To get old behavior of
nghttp2_session_get_stream_remote_window_size() is use
min(nghttp2_session_get_stream_remote_window_size(),
nghttp2_session_get_remote_window_size()).  The reason of this change
is that it is desirable to know just stream level window size without
taking into connection level window size.  This is useful for
debugging purpose.

Also edited its documentation to make hyperlink works.

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.