Callback function invoked to adjust priority value for request
HEADERS.

Since the application doesn’t know stream ID when it submits
requests, it may not be able to add correct priority value to HEADERS
frame and forced to use follwing PRIORITY frame. The purpose of this
callback is give the chance to the application to adjust priority
value with the latest information it has just before transmission so
that correct priority is included in HEADERS frame and it doesn’t
have to send additional PRIORITY frame.

The library interface supports compressed DATA.  The library does not
deflate nor inflate data payload.  When sending data, an application
has to compress data and set NGHTTP2_DATA_FLAG_COMPRESSED to
data_flags parameter in nghttp2_data_source_read_callback.  On
receiving, flags parameter in nghttp2_on_data_chunk_recv_callback
includes NGHTTP2_FLAG_COMPRESSED.  An application should check the
flags and inflate data as necessary.  Since compression context is per
frame, when DATA is seen in nghttp2_on_frame_recv_callback, an
application should reset compression context.

Handle the situation if a stream is told to depend on its descendant.
This is what
http://tools.ietf.org/html/draft-ietf-httpbis-http2-12#section-5.3.3
says.

If stream with dpri value of no_data, we check any its descendant has
stream with dpri value of top.  If so, we have to distribute of its
portion of weight to its descendants.

Previously h2load supports SPDY only for https URI.  This is because
SPDY has no mechanism to negotiate its protocol version without NPN.
With this change, user can specify the exact protocol version to use
when http URI (without SSL/TLS) is used.

It seems that setup.py gets deleted before python executes it in
clean-local.  To prevent this situation, we just use .NOTPARALLEL.

Previously we distribute nghttp2.c in python, which is cythonized C
source code from nghttp2.pyx.  Since it is distributed in archive it
exists in source directory.  But we use python distutils which
operates in build directory and does not support C source files in out
of tree directory (i.e., not under the build directory).  Copying C
source file to build directory is a bit dirty, so we just decided not
to ship nghttp2.c.