Assertion failure while streaming toasted data

Hi,

While working on an output plugin that uses streaming protocol, I hit an
assertion failure. Further investigations revealed a possible bug in core
Postgres. This must be new to PG14 since streaming support is new to this
release. I extended the test_decoding regression test to demonstrate the
failure. PFA

```
2021-05-25 11:32:19.493 IST client backend[68321] pg_regress/stream
STATEMENT: SELECT data FROM pg_logical_slot_get_changes('regression_slot',
NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '
1', 'stream-changes', '1');
TRAP: FailedAssertion("txn->size == 0", File: "reorderbuffer.c", Line:
3476, PID: 68321)
```

From my preliminary analysis, it looks like we fail to adjust the memory
accounting after streaming toasted tuples. More concretely, after
`ReorderBufferProcessPartialChange()` processes the in-progress
transaction, `ReorderBufferTruncateTXN()` truncates the accumulated
changed in the transaction, but fails to adjust the buffer size for toast
chunks. Maybe we are missing a call to `ReorderBufferToastReset()`
somewhere?

From what I see, the assertion only triggers when data is inserted via COPY
(multi-insert).

Let me know if anything else is needed to reproduce this.

Thanks,
Pavan

--
Pavan Deolasee
EnterpriseDB: https://www.enterprisedb.com