pgsql: Adjust pgstat_initstats() to avoid repeated searches of the

Log Message:
-----------
Adjust pgstat_initstats() to avoid repeated searches of the TabStat arrays
when a relation is opened multiple times in the same transaction.  This is
particularly useful for system catalogs, which we may heap_open or index_open
many times in a transaction, and it doesn't really cost anything extra even
if the rel is touched but once.  Motivated by study of an example from Greg
Stark, in which pgstat_initstats() accounted for an unreasonably large
fraction of the runtime.

Modified Files:
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    pgsql/src/backend/postmaster:
        pgstat.c (r1.152 -> r1.153)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.152&r2=1.153)