user callback. Updated doc.

spdylay_session_pack_data() now takes the maximum length of DATA payload.

SPDYLAY_ERR_ZLIB is now moved back to non-fatal error and it is subject
to GOAWAY if it occurred.

Fixed spdylay_frame_count_nv_space() bug. Check all data is processed in spdylay_frame_count_unpack_nv_space()

If SPDYLAY_ERR_ZLIB is encountered, zlib context is out of sync.
So we cannot further SYN_STREAM/SYN_REPLY/HEADERS frames.
I'm not sure we should send GOAWAY in this case.
So bail out with fatal error and drop connection for now.

Ensure that if flags contains SPDYLAY_FLAG_FIN, only the last DATA frame
has FLAG_FIN set.

Temporal name/value buffer will be shared by unpacking frame.