h2load: Don't use std::future with --disable-threads

Now we don't use std::future with --disable-threads, checking
std::future in configure.ac was removed and building h2load is always
enabled.
Document that only one data is allowed for one stream at a time
nghttpx: Fix request is sent to backend prematurely with http2 upstream
Distribute closed or blocked stream's weight to its siblings

This also means that at least one stream whose dpri is
NGHTTP2_STREAM_DPRI_TOP exists, its siblings descendants have no
chance to send streams, even if their parent stream has
NGHTTP2_STREAM_DPRI_NODATA.
Push stream to queue after effective weight was calculated
Define NOTHREADS if std:future is not available
src: Use fcntl and FD_CLOEXEC if O_CLOEXEC is undefined

We may run into race condition if execve is called at the same time
when fcntl is called.  But we just does this for now to compile
nghttp2 applications under older kernel.
examples: Disable tiny-nghttpd if timerfd_create is not available
Check first SETTINGS strictly
Update doc
Set payload length when expected SETTINGS is not received
Fix bugs found by coverity scan
Use switch-case instead of if
Bump up version number to 0.6.3, LT revision to 6:1:1
Bump up version number to 0.6.4-DEV