When trying IP addresses from the ticket, have a very rough sort order
in which to try them. Basically assume most local wifi's are
somewhere on 192.168.0.0/16 so prefer those first.
This replaces the mechanism by which the IoPauseGuard makes sure the
IO scheduler is resumed: it really is a drop guard now by sending a
single message on drop.
This makes it not have to hold on to anything like the context so
makes it a lot easier to use.
The trade-off is that a long-running task is spawned when the guard is
created, this task needs to receive the message from the drop guard in
order for the scheduler to resume.
This changes the JSON-RPC APIs to get a QR code from the backup
provider to block. It means once you have a (blocking) call to
provide_backup() you can call get_backup_qr() or get_backup_qr_svg()
and they will block until the QR code is available.
Calling get_backup_qr() or get_backup_qr_svg() when there is no backup
provider will immediately error.
This makes the BackupProvider automatically invoke pause-io while it
is needed.
It needed to make the guard independent from the Context lifetime to
make this work. Which is a bit sad.
This also removes BackupProvider::join in favour of implementing
Future directly. I wondered about implementing a FusedFutre to make
this a little safer but it would introduce a dependency on the futures
crate in deltachat-ffi which did not exist yet, so I didn't do that.
We no longer need that in the transfer case, that would give very
weird results. This also means there is nothing imex-specific about
this anymore so move it to blobs.rs
