We cleared the local 'useHeapMultiInsert' variable, as soon as we saw
at least one AO partition, even if we had already buffered tuples for
a heap partition earlier. As a result, we didn't flush the multi-insert
buffer at the end of the COPY.

Fixes https://github.com/greenplum-db/gpdb/issues/6678Reviewed-by: NAdam Lee <ali@pivotal.io>