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/ready.go

@@ -0,0 +1,37 @@
+package tdsync
+
+import (
+	"sync/atomic"
+)
+
+// Ready is simple signal primitive which sends signal once.
+// This is not allowed to use zero value.
+type Ready struct {
+	wait chan struct{}
+	done int32
+}
+
+// NewReady creates new Ready.
+func NewReady() *Ready {
+	return &Ready{
+		wait: make(chan struct{}),
+	}
+}
+
+func (r *Ready) reset() {
+	r.wait = make(chan struct{})
+	atomic.StoreInt32(&r.done, 0)
+}
+
+// Signal sends ready signal.
+// Can be called multiple times.
+func (r *Ready) Signal() {
+	if atomic.CompareAndSwapInt32(&r.done, 0, 1) {
+		close(r.wait)
+	}
+}
+
+// Ready returns waiting channel.
+func (r *Ready) Ready() <-chan struct{} {
+	return r.wait
+}