Add "resume only" option (not creating any snapshots)

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(originally by @diraimondo)

I'm wondering if it would be possible to add an option (command-line and/or config) to let btrbk attempt to transfer all the already-created snapshots on source that are missing on target. This skipping any attempt to create further snaps.
This would fit in the following scenario:

• a laptop where I periodically and/or manually create local snaps (like a local time-machine); this could happens even when away from home for many days; the configuration would lack any target and could keep, for example, at most 10 snaps to save space on laptop HD;
• a server at home that frequently, every hour, try connect the laptop (if present). If there exist at least a new snap on laptop that are not present on server, it should be transferred. In this case it would be sufficient to get just the most recent, not all new snaps. (this looks the missing feature).

Thank you for your time.

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In this scenario, we have two instances of btrbk:

1. Laptop instance, run periodically and/or manually. Dedicated to create snapshots only.
2. Fileserver instance, run periodically. Dedicated to grab snapshots created by another instance.

A configuration would look like this:

Laptop config:

volume /mnt/btr_pool
  subvolume home
    snapshot_dir             btrbk_snapshots
    snapshot_create_always   yes

Fileserver config:

volume ssh://my-laptop.com/mnt/btr_pool
  subvolume home
    snapshot_dir             btrbk_snapshots
    snapshot_preserve_daily  all   <-- don't delete any snapshots
    snapshot_create          no    <-- new feature requested
    resume_missing           yes   <-- resume according to "target_preserve_*" options

    target send-receive  /mnt/btr_backup/my-laptop.com

The configuration above assumes a new (not yet implemented) option snapshot_create no.
The configuration option snapshot_create_always will be deprecated in favor of:

• snapshot_create always: same behaviour as deprecated snapshot_create_always yes
• snapshot_create ondemand: same behaviour as deprecated snapshot_create_always no
• snapshot_create no: new feature, never create any snapshots

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Implemented in: 7251915

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Included in v0.18.0