pgsql: Optimise btree delete processing when no active backends.

Log Message:
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Optimise btree delete processing when no active backends.
Clarify comments, downgrade a message to DEBUG and remove some
debug counters. Direct from ideas by Heikki Linnakangas.

Modified Files:
--------------
    pgsql/src/backend/access/nbtree:
        nbtxlog.c (r1.66 -> r1.67)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtxlog.c?r1=1.66&r2=1.67)
    pgsql/src/backend/storage/ipc:
        procarray.c (r1.65 -> r1.66)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c?r1=1.65&r2=1.66)
        standby.c (r1.17 -> r1.18)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/standby.c?r1=1.17&r2=1.18)