config: support zrepl's day and week units for snapshotting.interval

Originally, I had a patch that would replace all usages of
time.Duration in package config with the new config.Duration
types, but:
1. these are all timeouts/retry intervals that have default values.
   Most users don't touch them, and if they do, they don't need
   day or week units.
2. go-yaml's error reporting for yaml.Unmarshaler is inferior to
   built-in types (line numbers are missing, so the error would not have
   sufficient context)

fixes https://github.com/zrepl/zrepl/issues/486

daemon/snapper/periodic.go

@@ -22,7 +22,7 @@ func periodicFromConfig(g *config.Global, fsf zfs.DatasetFilter, in *config.Snap
 	if in.Prefix == "" {
 		return nil, errors.New("prefix must not be empty")
 	}
-	if in.Interval <= 0 {
+	if in.Interval.Duration() <= 0 {
 		return nil, errors.New("interval must be positive")
 	}
 
@@ -32,7 +32,7 @@ func periodicFromConfig(g *config.Global, fsf zfs.DatasetFilter, in *config.Snap
 	}
 
 	args := periodicArgs{
-		interval: in.Interval,
+		interval: in.Interval.Duration(),
 		fsf:      fsf,
 		planArgs: planArgs{
 			prefix:          in.Prefix,