feature: send-receive to multiple receive targets at once

Instead of having in my `btrbk.conf`, e.g.
```
target send-receive /mnt/dest1
target send-receive /mnt/dest2
```
it would be good if by specifying
```
target send-receive /mnt/dest1 /mnt/dest2
```
`btrbk` would use `btrfs send` to send to dest1 and dest2 simultaneously, i.e., do something like this:
```
btrfs send /tmp/incremental_backup_snapshot | tee >(btrfs receive /mnt/dest1) | btrfs receive /mnt/dest2
```
^{(cf. "[How do I send btrfs snapshots to multiple destination drives?](https://stackoverflow.com/q/48234297/1429450)")}
This would save time when making mirrored backups on multiple drives.

Start with the target syntax shown in btrbk.conf and trace how btrbk invokes btrfs send and btrfs receive. The feature is done when one target send-receive entry can name /mnt/dest1 and /mnt/dest2 and send the snapshot to both destinations simultaneously.

Written by the indexing model from the issue text.