snapshotting: ability to specify timestamp location != UTC (#801)

This PR adds a new field optional field `timestamp_location` that allows
the user to specify a timezone different than the default UTC for use in
the snapshot suffix.

I took @mjasnik 's PR https://github.com/zrepl/zrepl/pull/785 and
refactored+extended it as follows:
* move all formatting logic into its own package
* disallow `dense` and `human` with formats != UTC to protect users from
stupidity
* document behavior more clearly
* regression test for existing users

daemon/snapper/cron.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/hooks"
+	"github.com/zrepl/zrepl/daemon/snapper/snapname"
 	"github.com/zrepl/zrepl/util/suspendresumesafetimer"
 	"github.com/zrepl/zrepl/zfs"
 )
@@ -20,10 +21,15 @@ func cronFromConfig(fsf zfs.DatasetFilter, in config.SnapshottingCron) (*Cron, e
 	if err != nil {
 		return nil, errors.Wrap(err, "hook config error")
 	}
+
+	formatter, err := snapname.New(in.Prefix, in.TimestampFormat, in.TimestampLocation)
+	if err != nil {
+		return nil, errors.Wrap(err, "build snapshot name formatter")
+	}
+
 	planArgs := planArgs{
-		prefix:          in.Prefix,
-		timestampFormat: in.TimestampFormat,
-		hooks:           hooksList,
+		formatter: formatter,
+		hooks:     hooksList,
 	}
 	return &Cron{config: in, fsf: fsf, planArgs: planArgs}, nil
 }