SoupSessionSync: fix messages getting stuck forever

SoupSessionSync: fix messages getting stuck forever

Messages in SoupSessionSync were getting stuck forever if they didn't
manage to get a connection on the first try; the test programs didn't
pick this up because none of them queue enough messages all at once to
hit the max-conns limit.

Fix the messages-getting-stuck bug, and add a test case to misc-test
to verify it. Also fix another SoupSessionSync bug where cancelling
several messages at once could cause spurious errors.

https://bugzilla.gnome.org/show_bug.cgi?id=634422