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

config/config_snapshotting_test.go

@@ -163,7 +163,8 @@ jobs:
 		assert.Equal(t, "periodic", snp.Type)
 		assert.Equal(t, 10*time.Minute, snp.Interval.Duration())
 		assert.Equal(t, "zrepl_", snp.Prefix)
-		assert.Equal(t, "dense", snp.TimestampFormat) // default was set correctly
+		assert.Equal(t, snp.TimestampFormat, "dense")
+		assert.Equal(t, snp.TimestampLocation, "UTC")
 	})
 
 	t.Run("cron", func(t *testing.T) {
@@ -171,6 +172,7 @@ jobs:
 		snp := c.Jobs[0].Ret.(*PushJob).Snapshotting.Ret.(*SnapshottingCron)
 		assert.Equal(t, "cron", snp.Type)
 		assert.Equal(t, "zrepl_", snp.Prefix)
-		assert.Equal(t, "dense", snp.TimestampFormat) // default was set correctly
+		assert.Equal(t, snp.TimestampFormat, "dense")
+		assert.Equal(t, snp.TimestampLocation, "UTC")
 	})
 }