WIP: recurring jobs

Done:

* implement autosnapper that asserts interval between snapshots
* implement pruner

* job pull: pulling + pruning
* job source: autosnapping + serving

TODO

* job source: pruning
* job local: everything
* fatal errors such as serve that cannot bind socket must be more
visible
* couldn't things that need a snapshotprefix just use a interface
Prefixer() instead? then we could have prefixsnapshotfilter and not
duplicate it every time...
* either go full context.Context or not at all...? just wait because
community climate around it isn't that great and we only need it for
cancellation? roll our own?

cmd/sampleconf/pullbackup/backuphost.yml

@@ -9,6 +9,10 @@ jobs:
     port: 22
     identity_file: /root/.ssh/id_ed25519
 
+  # pull (=ask for new snapshots) every 10m, prune afterwards
+  # this will leave us at most 10m behind production
+  interval: 10m
+
   # pull all offered filesystems to storage/backups/zrepl/pull/prod1.example.com
   mapping: {
     "<":"storage/backups/zrepl/pull/prod1.example.com"