Remove incorrect assertion in reorderbuffer.c.

Remove incorrect assertion in reorderbuffer.c.

We start recording changes in ReorderBufferTXN even before we reach
SNAPBUILD_CONSISTENT state so that if the commit is encountered after
reaching that we should be able to send the changes of the entire transaction.
Now, while recording changes if the reorder buffer memory has exceeded
logical_decoding_work_mem then we can start streaming if it is allowed and
we haven't yet streamed that data. However, we must not allow streaming to
start unless the snapshot has reached SNAPBUILD_CONSISTENT state.

In passing, improve the comments atop ReorderBufferResetTXN to mention the
case when we need to continue streaming after getting an error.

Author: Amit Kapila
Reviewed-by: Dilip Kumar
Discussion: http://postgr.es/m/CAA4eK1KoOH0byboyYY40NBcC7Fe812trwTa+WY3jQF7WQWZbQg@mail.gmail.com