soup-session: don't mark CONNECTING connections IDLE on cancel

soup-session: don't mark CONNECTING connections IDLE on cancel

When finishing up a queue item, SoupSession was always marking its
connection IDLE, even if it had previously been CONNECTING. This
created a race condition where if lots of pending connections were
cancelled at once, some of them might see connections labelled IDLE
that didn't actually have a SoupSocket (before those connections got
asynchronously cleaned up). Fix this by only marking connections IDLE
if they had previously been IN_USE.

https://bugzilla.gnome.org/show_bug.cgi?id=667245