pgsql: Teach bitmap heap scan to cope with absence of a DSA.

Teach bitmap heap scan to cope with absence of a DSA. If we have a plan that uses parallelism but are unable to execute it using parallelism, for example due to a lack of available DSM segments, then the EState's es_query_dsa will be NULL. Parallel bitmap heap scan needs to fall back to a non-par

pgsql: Teach bitmap heap scan to cope with absence of a DSA.

Teach bitmap heap scan to cope with absence of a DSA. If we have a plan that uses parallelism but are unable to execute it using parallelism, for example due to a lack of available DSM segments, then the EState's es_query_dsa will be NULL. Parallel bitmap heap scan needs to fall back to a non-par