sync job: pull: improve log output

sync job: pull: improve log output

Adding an opening line for every group makes parsing the log easier.

We can also remove the 're-sync [...] done' line, because the next
line should be a progress line anyway.

The new output for the sync job/pull logs looks as follows:

- skipped already synced (happens in most jobs, except for first run)
- re-sync of last synced snapshot (if it still exists on source)
- skipped because of transfer-last (if set and skips something)
- sync of new snapshots (if they exist)

Suggested-By: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>