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/snapname/snapname.go

@@ -0,0 +1,52 @@
+package snapname
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/pkg/errors"
+
+	"github.com/zrepl/zrepl/daemon/snapper/snapname/timestamp"
+	"github.com/zrepl/zrepl/zfs"
+)
+
+type Formatter struct {
+	prefix    string
+	timestamp *timestamp.Formatter
+}
+
+func New(prefix, tsFormat, tsLocation string) (*Formatter, error) {
+	timestamp, err := timestamp.New(tsFormat, tsLocation)
+	if err != nil {
+		return nil, errors.Wrap(err, "build timestamp formatter")
+	}
+	formatter := &Formatter{
+		prefix:    prefix,
+		timestamp: timestamp,
+	}
+	// Best-effort check to detect whether the result would be an invalid name.
+	// Test two dates that in most places have will have different time zone offsets due to DST.
+	check := func(t time.Time) error {
+		testFormat := formatter.Format(t)
+		if err := zfs.ComponentNamecheck(testFormat); err != nil {
+			// testFormat last, can be quite long
+			return fmt.Errorf("`invalid snapshot name would result from `prefix+$timestamp`: %s: %q", err, testFormat)
+		}
+		return nil
+	}
+	if err := check(time.Date(2020, 6, 1, 0, 0, 0, 0, time.UTC)); err != nil {
+		return nil, err
+	}
+	if err := check(time.Date(2020, 12, 1, 0, 0, 0, 0, time.UTC)); err != nil {
+		return nil, err
+	}
+	return formatter, nil
+}
+
+func (f *Formatter) Format(now time.Time) string {
+	return f.prefix + f.timestamp.Format(now)
+}
+
+func (f *Formatter) Prefix() string {
+	return f.prefix
+}