By setting this option to 1 libcurl will wait for a connection to reveal
if it is possible to pipeline/multiplex on before it continues.

... and suddenly things work much better!

... so that they'll get handled next in the multi loop.

to allow code to act differently on the situation.

Also added some more info message for the connection re-use function to
make it clearer when connections are not re-used.

It makes us use less memory when not doing HTTP/2 and subsequently also
makes us not have to cleanup HTTP/2 related data when not using HTTP/2!

Previously when we do pause because of out of buffer, we just throw
away unread data in connection buffer.  This just broke protocol
framing, and I saw occasional FRAME_SIZE_ERROR.  This commit fix this
issue by remembering how much data read, and in the next iteration, we
process remaining data.

This commit fixes the bug that streams get stuck if stream gets some
DATA, and stream->closed becomes true at the same time.  Previously,
in this condition, after we processed DATA, we are going to try to
read data from underlying transport, but there is no data, and gets
EAGAIN.  There was no code path to evaludate stream->closed.

... as it was only used from there.

With the "drained" functionality we can get here slightly asynchronously
so the stream have have been closed but there is pending data left to
read.

... as it does for pipelining when we're multiplexing, as we need the
different buffers to store incoming data correctly for all streams.

No need to wait for our "spot" like for pipelining

... which is necessary since the socket won't be readable but there is
data waiting in the buffer.

... from the connection struct. The stream one being the 'struct HTTP'
which is kept in the SessionHandle struct (easy handle).

lookup streams for incoming frames in the stream hash, hashing is based
on the stream id and we get the SessionHandle for the incoming stream
that way.

Once we know we are HTTP/2 enabled we know the server can multiplex.

... and do not blacklist any.

All the details mentioned here are better documented in man pages