SoupSession: bind connections to messages sooner, improve conn failure case

SoupSession: bind connections to messages sooner, improve conn failure case

Rather than having connections be independent until a message is
actually sent on them, assign them to queue items when they're created
for them. This means that, eg, if another connection frees up while
we're waiting for the first connection to connect, that we can't take
advantage of that. But it simplifies a ton of other things.

Also, if a new connection fails, don't fail every other message on
that host; let them succeed or fail independently. When doing a
CONNECT though, keep track of the queue item that necessitated it so
we can error that out if the CONNECT fails, just like we would if the
actual soup_connection_connect() had failed.

https://bugzilla.gnome.org/show_bug.cgi?id=619633