This is done by making sure each individual transfer is first added to a
linked list as then they can be performed serially, or at will, in
parallel.
Closes#3804
By properly keeping track of the last entry in the list of URLs/uploads
to handle, curl now avoids many meaningless traverses of the list which
speeds up many-URL handling *MASSIVELY* (several magnitudes on 100K
URLs).
Added test 1291, to verify that it doesn't take ages - but we don't have
any detection of "too slow" command in the test suite.
Reported-by: arainchik on github
Fixes#1959Closes#2052
