Use txn writer to write schema postings (#4296)

Fixes #3916

Although I was not able to reproduce this. Stacktrace suggests, while writing schema to Badger, it 
got crashed with error: Txn is too big to fit into one request. This might be because, we are using single transaction to write schema for all predicates.
This PR, replaces single transaction with TxnWriter to write schema.

dgraph/cmd/bulk/schema.go

@@ -143,8 +143,7 @@ func (s *schemaStore) getPredicates(db *badger.DB) []string {
 }
 
 func (s *schemaStore) write(db *badger.DB, preds []string) {
-	txn := db.NewTransactionAt(math.MaxUint64, true)
-	defer txn.Discard()
+	w := posting.NewTxnWriter(db)
 	for _, pred := range preds {
 		sch, ok := s.schemaMap[pred]
 		if !ok {
@@ -153,25 +152,18 @@ func (s *schemaStore) write(db *badger.DB, preds []string) {
 		k := x.SchemaKey(pred)
 		v, err := sch.Marshal()
 		x.Check(err)
-		x.Check(txn.SetEntry(&badger.Entry{
-			Key:      k,
-			Value:    v,
-			UserMeta: posting.BitSchemaPosting}))
+		// Write schema and types always at timestamp 1, s.state.writeTs may not be equal to 1
+		// if bulk loader was restarted or other similar scenarios.
+		x.Check(w.SetAt(k, v, posting.BitSchemaPosting, 1))
 	}
 
 	// Write all the types as all groups should have access to all the types.
 	for _, typ := range s.types {
 		k := x.TypeKey(typ.TypeName)
 		v, err := typ.Marshal()
 		x.Check(err)
-		x.Check(txn.SetEntry(&badger.Entry{
-			Key:      k,
-			Value:    v,
-			UserMeta: posting.BitSchemaPosting,
-		}))
+		x.Check(w.SetAt(k, v, posting.BitSchemaPosting, 1))
 	}
 
-	// Write schema always at timestamp 1, s.state.writeTs may not be equal to 1
-	// if bulk loader was restarted or other similar scenarios.
-	x.Check(txn.CommitAt(1, nil))
+	x.Check(w.Flush())
 }