url: Fix CURLOPT_MAXAGE_CONN time comparison

Old connections are meant to expire from the connection cache after
CURLOPT_MAXAGE_CONN seconds. However, they actually expire after 1000x
that value. This occurs because a time value measured in milliseconds is
accidentally divided by 1M instead of by 1,000.

Closes https://github.com/curl/curl/pull/4013