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

zfs/namecheck_test.go

@@ -49,6 +49,10 @@ func TestEntityNamecheck(t *testing.T) {
 		{strings.Repeat("a", MaxDatasetNameLen-2) + "/a", EntityTypeFilesystem, true},
 		{strings.Repeat("a", MaxDatasetNameLen-4) + "/a@b", EntityTypeSnapshot, true},
 		{strings.Repeat("a", MaxDatasetNameLen) + "/a@b", EntityTypeSnapshot, false},
+		// + is not allowed, and particularly relevant to test here because
+		// common timestamp formats usually use `+` as a delimiter for numeric timezone offset
+		// => cf with package `timestamp_formatting`
+		{"foo/bar@23+42", EntityTypeSnapshot, false},
 	}
 
 	for idx := range tcs {