Reinstate the MTA-STS (Strict Transport Security) implementation

This commit brings back the experimental MTA-STS (Strict Transport
Security) implementation, removed in commit
7f5bedf4aa.

We will continue development in the "sts" branch, subject to rebase,
until it is ready to be integrated into "next" again.

chasquid.go

@@ -7,6 +7,7 @@
 package main
 
 import (
+	"context"
 	"expvar"
 	"flag"
 	"fmt"
@@ -25,6 +26,7 @@ import (
 	"blitiri.com.ar/go/chasquid/internal/maillog"
 	"blitiri.com.ar/go/chasquid/internal/normalize"
 	"blitiri.com.ar/go/chasquid/internal/smtpsrv"
+	"blitiri.com.ar/go/chasquid/internal/sts"
 	"blitiri.com.ar/go/chasquid/internal/userdb"
 	"blitiri.com.ar/go/log"
 	"blitiri.com.ar/go/systemd"
@@ -146,12 +148,18 @@ func main() {
 
 	dinfo := s.InitDomainInfo(conf.DataDir + "/domaininfo")
 
+	stsCache, err := sts.NewCache(conf.DataDir + "/sts-cache")
+	if err != nil {
+		log.Fatalf("Failed to initialize STS cache: %v", err)
+	}
+	go stsCache.PeriodicallyRefresh(context.Background())
+
 	localC := &courier.Procmail{
 		Binary:  conf.MailDeliveryAgentBin,
 		Args:    conf.MailDeliveryAgentArgs,
 		Timeout: 30 * time.Second,
 	}
-	remoteC := &courier.SMTP{Dinfo: dinfo}
+	remoteC := &courier.SMTP{Dinfo: dinfo, STSCache: stsCache}
 	s.InitQueue(conf.DataDir+"/queue", localC, remoteC)
 
 	// Load the addresses and listeners.