h2load has supported uploading a file quite a while, but it turns out
that it worked with HTTP/2 and SPDY only. HTTP/1 with upload did not
work. This commit fixes this bug, and implement HTTP/1 upload. Due
to architectural limitation of h2load, when -d option is used, the
number of in-flight pipe-lined requests is set to 1.
The profiler and benchmarking showed that calling evbuffer_add()
repeatedly is very costly. To avoid this, we buffer up small writes
into one large chunk and call evbuffer_add() less times.