pgsql: Further reductions in Hot Standby conflict processing.

Log Message:
-----------
Further reductions in Hot Standby conflict processing. These
come from the realistion that HEAP2_CLEAN records don't
always remove user visible data, so conflict processing for
them can be skipped. Confirm validity using Assert checks,
clarify circumstances under which we log heap_cleanup_info
records. Tuning arises from bug fixing of earlier safety
check failures.

Modified Files:
--------------
pgsql/src/backend/access/heap:
heapam.c (r1.288 -> r1.289)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.288&r2=1.289)
pruneheap.c (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/pruneheap.c?r1=1.23&r2=1.24)
pgsql/src/backend/commands:
vacuumlazy.c (r1.134 -> r1.135)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.134&r2=1.135)