curl: speed up handling of many URLs

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 #1959
Closes #2052