Fix assertion during streaming of multi-insert toast changes.

While decoding the multi-insert WAL we can't clean the toast untill we get
the last insert of that WAL record. Now if we stream the changes before we
get the last change, the memory for toast chunks won't be released and we
expect the txn to have streamed all changes after streaming.  This
restriction is mainly to ensure the correctness of streamed transactions
and it doesn't seem worth uplifting such a restriction just to allow this
case because anyway we will stream the transaction once such an insert is
complete.

Previously we were using two different flags (one for toast tuples and
another for speculative inserts) to indicate partial changes. Now instead
we replaced both of them with a single flag to indicate partial changes.

Reported-by: Pavan Deolasee
Author: Dilip Kumar
Reviewed-by: Pavan Deolasee, Amit Kapila
Discussion: https://postgr.es/m/CABOikdN-_858zojYN-2tNcHiVTw-nhxPwoQS4quExeweQfG1Ug@mail.gmail.com

contrib/test_decoding/expected/stream.out

@@ -82,6 +82,30 @@ SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL,NULL, 'incl
  committing streamed transaction
 (13 rows)
 
+-- streaming test for toast with multi-insert
+\COPY stream_test FROM STDIN
+SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL,NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
+                   data                   
+------------------------------------------
+ opening a streamed block for transaction
+ streaming change for transaction
+ streaming change for transaction
+ streaming change for transaction
+ streaming change for transaction
+ streaming change for transaction
+ streaming change for transaction
+ streaming change for transaction
+ streaming change for transaction
+ streaming change for transaction
+ streaming change for transaction
+ streaming change for transaction
+ closing a streamed block for transaction
+ opening a streamed block for transaction
+ streaming change for transaction
+ closing a streamed block for transaction
+ committing streamed transaction
+(17 rows)
+
 DROP TABLE stream_test;
 SELECT pg_drop_replication_slot('regression_slot');
  pg_drop_replication_slot