pgsql: Fix oversight in collecting values for cleanup_info records.

Log Message:
-----------
Fix oversight in collecting values for cleanup_info records.
vacuum_log_cleanup_info() now generates log records with a valid
latestRemovedXid set in all cases. Also be careful not to zero the
value when we do a round of vacuuming part-way through lazy_scan_heap().
Incidentally, this reduces frequency of conflicts in Hot Standby.

Modified Files:
--------------
pgsql/src/backend/access/heap:
pruneheap.c (r1.22 -> r1.23)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/pruneheap.c?r1=1.22&r2=1.23)
pgsql/src/backend/commands:
vacuumlazy.c (r1.132 -> r1.133)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.132&r2=1.133)
pgsql/src/include/access:
heapam.h (r1.148 -> r1.149)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/heapam.h?r1=1.148&r2=1.149)