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.