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/tdsync/backoff.go

@@ -0,0 +1,31 @@
+package tdsync
+
+import (
+	"sync"
+	"time"
+
+	"github.com/cenkalti/backoff/v4"
+)
+
+type syncBackoff struct {
+	b   backoff.BackOff
+	mux sync.Mutex
+}
+
+func (s *syncBackoff) NextBackOff() time.Duration {
+	s.mux.Lock()
+	dur := s.b.NextBackOff()
+	s.mux.Unlock()
+	return dur
+}
+
+func (s *syncBackoff) Reset() {
+	s.mux.Lock()
+	s.b.Reset()
+	s.mux.Unlock()
+}
+
+// SyncBackoff decorates backoff.BackOff to be thread-safe.
+func SyncBackoff(from backoff.BackOff) backoff.BackOff {
+	return &syncBackoff{b: from}
+}