HTTP/2: reworked body reading to better match HTTP/1.x code.

HTTP/2: reworked body reading to better match HTTP/1.x code.

In particular, now the code always uses a buffer limited by
client_body_buffer_size.  At the cost of an additional copy it
ensures that small DATA frames are not directly mapped to small
write() syscalls, but rather buffered in memory before writing.
Further, requests without Content-Length are no longer forced
to use temporary files.