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/periodic.go

@@ -10,6 +10,7 @@ import (
 	"github.com/pkg/errors"
 
 	"github.com/zrepl/zrepl/daemon/logging/trace"
+	"github.com/zrepl/zrepl/daemon/snapper/snapname"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/hooks"
@@ -32,13 +33,17 @@ func periodicFromConfig(g *config.Global, fsf zfs.DatasetFilter, in *config.Snap
 		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")
+	}
+
 	args := periodicArgs{
 		interval: in.Interval.Duration(),
 		fsf:      fsf,
 		planArgs: planArgs{
-			prefix:          in.Prefix,
-			timestampFormat: in.TimestampFormat,
-			hooks:           hookList,
+			formatter: formatter,
+			hooks:     hookList,
 		},
 		// ctx and log is set in Run()
 	}
@@ -166,7 +171,7 @@ func periodicStateSyncUp(a periodicArgs, u updater) state {
 	if err != nil {
 		return onErr(err, u)
 	}
-	syncPoint, err := findSyncPoint(a.ctx, fss, a.planArgs.prefix, a.interval)
+	syncPoint, err := findSyncPoint(a.ctx, fss, a.planArgs.formatter.Prefix(), a.interval)
 	if err != nil {
 		return onErr(err, u)
 	}