SPF Optimizer: Enable the use of TXTMulti records to support longer SPF records (#794)

* Add multiple string support to SPF optimizer

Notes:

* This implements [RFC 4408][rfc] for the SPF optimizer. Allowing for
more SPF records to fit within the 10 lookups by using multiple strings.
* By default the max size of the TXT remains at 255. Meaning users will
still only get a single 255 length string unless they modify `txtMaxSize`
and opt into this feature.
* The general recommendation when using multiple strings for TXT records
is to keep the size within a single UDP packet. It seems like the
maximum size for this depends on a bunch of factors that are sometimes
outside of your control. A similar tool has a [formula for estimating the
maximum allowed size][formula]. However I felt giving a user
configurable size would fit with the current configuration style that
dnscontrol has. Similar to how dnscontrol recommends only flattening a
record if absolutely needed, I can see this length being increased by
only enough to get you within 10 lookups.

[rfc]: https://tools.ietf.org/html/rfc4408#section-3.1.3
[formula]: https://github.com/oasys/mkspf/blob/master/Overhead.md

* Add a nice comment for the Chunks function

pkg/normalize/flatten.go

@@ -58,17 +58,27 @@ func flattenSPFs(cfg *models.DNSConfig) []error {
 					overhead1 = i
 				}
 
+				// Default txtMaxSize will not result in multiple TXT strings
+				txtMaxSize := 255
+				if oh, ok := txt.Metadata["txtMaxSize"]; ok {
+					i, err := strconv.Atoi(oh)
+					if err != nil {
+						errs = append(errs, Warning{fmt.Errorf("split txtMaxSize %q is not an int", oh)})
+					}
+					txtMaxSize = i
+				}
+
 				if !strings.Contains(split, "%d") {
 					errs = append(errs, Warning{fmt.Errorf("Split format `%s` in `%s` is not proper format (should have %%d in it)", split, txt.GetLabelFQDN())})
 					continue
 				}
-				recs := rec.TXTSplit(split+"."+domain.Name, overhead1)
+				recs := rec.TXTSplit(split+"."+domain.Name, overhead1, txtMaxSize)
 				for k, v := range recs {
 					if k == "@" {
-						txt.SetTargetTXT(v)
+						txt.SetTargetTXTs(v)
 					} else {
 						cp, _ := txt.Copy()
-						cp.SetTargetTXT(v)
+						cp.SetTargetTXTs(v)
 						cp.SetLabelFromFQDN(k, domain.Name)
 						domain.Records = append(domain.Records, cp)
 					}