pgsql: Augment WAL records for btree delete with GetOldestXmin() to

Log Message:
-----------
Augment WAL records for btree delete with GetOldestXmin() to reduce
false positives during Hot Standby conflict processing. Simple
patch to enhance conflict processing, following previous discussions.
Controlled by parameter minimize_standby_conflicts = on | off, with
default off allows measurement of performance impact to see whether
it should be set on all the time.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.246 -> r1.247)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.246&r2=1.247)
    pgsql/src/backend/access/nbtree:
        nbtpage.c (r1.115 -> r1.116)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtpage.c?r1=1.115&r2=1.116)
    pgsql/src/backend/access/transam:
        xlog.c (r1.364 -> r1.365)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.364&r2=1.365)
    pgsql/src/backend/utils/misc:
        guc.c (r1.536 -> r1.537)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.536&r2=1.537)
        postgresql.conf.sample (r1.274 -> r1.275)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.274&r2=1.275)
    pgsql/src/include/access:
        xlog.h (r1.99 -> r1.100)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h?r1=1.99&r2=1.100)