move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/mtproto/salt.go

@@ -0,0 +1,74 @@
+package mtproto
+
+import (
+	"context"
+	"time"
+
+	"github.com/go-faster/errors"
+	"go.uber.org/zap"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/mt"
+)
+
+func (c *Conn) storeSalt(salt int64) {
+	c.sessionMux.Lock()
+	// Copy to log.
+	oldSalt := c.salt
+	c.salt = salt
+	c.sessionMux.Unlock()
+
+	if salt != oldSalt {
+		c.log.Info("Salt updated", zap.Int64("old", oldSalt), zap.Int64("new", salt))
+	}
+}
+
+func (c *Conn) updateSalt() {
+	salt, ok := c.salts.Get(c.clock.Now().Add(time.Minute * 5))
+	if !ok {
+		return
+	}
+
+	c.storeSalt(salt)
+}
+
+const defaultSaltsNum = 4
+
+func (c *Conn) getSalts(ctx context.Context) error {
+	request := &mt.GetFutureSaltsRequest{
+		Num: defaultSaltsNum,
+	}
+	ctx, cancel := context.WithTimeout(ctx, c.getTimeout(request.TypeID()))
+	defer cancel()
+
+	if err := c.writeServiceMessage(ctx, request); err != nil {
+		return errors.Wrap(err, "request salts")
+	}
+
+	return nil
+}
+
+func (c *Conn) saltLoop(ctx context.Context) error {
+	select {
+	case <-c.gotSession.Ready():
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+
+	// Get salts first time.
+	if err := c.getSalts(ctx); err != nil {
+		return err
+	}
+
+	ticker := c.clock.Ticker(c.saltFetchInterval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ticker.C():
+			if err := c.getSalts(ctx); err != nil {
+				return err
+			}
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	}
+}