Add "resume only" option (not creating any snapshots) about btrbk HOT 3 CLOSED

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

from btrbk.

Implemented in: 7251915

from btrbk.

Included in v0.18.0

from btrbk.