if the first smtp or imap command is invalid, shut down the connection instead of trying to read more

this is quite common on the internet. the other side may be trying some other
protocol, e.g. http, or some common vulnerability. we don't want to spam our
own logs with multiple invalid lines. if the first command is valid, but later
are not, we'll keep trying to process them. so this only affects protocol
sessions that are very likely not smtp/imap.

also remove a few more sleeps during tests, making imapserver and smtpserver tests a bit faster.

imapserver/fuzz_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/base64"
 	"errors"
 	"fmt"
+	"io"
 	"net"
 	"os"
 	"testing"
@@ -106,7 +107,7 @@ func FuzzServer(f *testing.F) {
 						return
 					}
 					err, ok := x.(error)
-					if !ok || !errors.Is(err, os.ErrDeadlineExceeded) {
+					if !ok || (!errors.Is(err, os.ErrDeadlineExceeded) && !errors.Is(err, io.EOF)) {
 						panic(x)
 					}
 				}()