Starting with this change when doing parallel transfers, without this
option set, curl will prefer to create new transfers multiplexed on an
existing connection rather than creating a brand new one.
--parallel-immediate can be set to tell curl to prefer to use new
connections rather than to wait and try to multiplex.
libcurl-wise, this means that curl will set CURLOPT_PIPEWAIT by default
on parallel transfers.
Suggested-by: Tom van der Woerdt
Closes#4500
