HSTS amnesia with --parallel

curl’s HSTS cache saving behaves wrongly when multiple URLs are requested in
parallel.

Using its HSTS support, curl can be instructed to use HTTPS instead of using
an insecure clear-text HTTP step even when HTTP is provided in the URL. This
HSTS mechanism would however surprisingly fail when multiple transfers are done
in parallel as the HSTS cache file gets overwritten by the most recently
completed transfer.

A later HTTP-only transfer to the earlier hostname would then not get
upgraded properly to HSTS.

Reproducible like this:

1. curl --hsts hsts.txt --parallel https://curl.se https://example.com
2. curl --hsts hsts.txt http://curl.se