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/salts/salts.go

@@ -0,0 +1,90 @@
+// Package salts contains MTProto server salt storage.
+package salts
+
+import (
+	"sort"
+	"sync"
+	"time"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/mt"
+)
+
+// Salts is a simple struct store server salts.
+type Salts struct {
+	// server salts fetched by getSalts.
+	salts    []mt.FutureSalt
+	saltsMux sync.Mutex
+}
+
+// Get returns next valid salt.
+func (s *Salts) Get(deadline time.Time) (int64, bool) {
+	s.saltsMux.Lock()
+	defer s.saltsMux.Unlock()
+
+check:
+	if len(s.salts) < 1 {
+		return 0, false
+	}
+
+	date := int(deadline.Unix())
+	if salt := s.salts[len(s.salts)-1]; salt.ValidUntil > date {
+		return salt.Salt, true
+	}
+
+	// Filter (in place) from SliceTricks.
+	n := 0
+	// Check that the salt will be valid until deadline.
+	for _, salt := range s.salts {
+		// Filter expired salts.
+		if salt.ValidUntil > date {
+			// Keep valid salt.
+			s.salts[n] = salt
+			n++
+		}
+	}
+	s.salts = s.salts[:n]
+	goto check
+}
+
+type saltSlice []mt.FutureSalt
+
+func (s saltSlice) Len() int {
+	return len(s)
+}
+
+func (s saltSlice) Less(i, j int) bool {
+	return s[i].ValidUntil > s[j].ValidUntil
+}
+
+func (s saltSlice) Swap(i, j int) {
+	s[i], s[j] = s[j], s[i]
+}
+
+// Store stores all given salts.
+func (s *Salts) Store(salts []mt.FutureSalt) {
+	s.saltsMux.Lock()
+	defer s.saltsMux.Unlock()
+
+	s.salts = append(s.salts, salts...)
+	// Filter duplicates.
+	n := 0
+	dedup := make(map[int64]struct{}, len(s.salts)+1)
+	for _, salt := range s.salts {
+		if _, ok := dedup[salt.Salt]; !ok {
+			dedup[salt.Salt] = struct{}{}
+			s.salts[n] = salt
+			n++
+		}
+	}
+	s.salts = s.salts[:n]
+
+	// Sort slice by valid until.
+	sort.Sort(saltSlice(s.salts))
+}
+
+// Reset deletes all stored salts.
+func (s *Salts) Reset() {
+	s.saltsMux.Lock()
+	s.salts = s.salts[:0]
+	s.saltsMux.Unlock()
+}