Fix things so that when CREATE INDEX CONCURRENTLY sets pg_index.indisvalid

Fix things so that when CREATE INDEX CONCURRENTLY sets pg_index.indisvalid
true at the very end of its processing, the update is broadcast via a
shared-cache-inval message for the index; without this, existing backends that
already have relcache entries for the index might never see it become valid.
Also, force a relcache inval on the index's parent table at the same time,
so that any cached plans for that table are re-planned; this ensures that
the newly valid index will be used if appropriate.  Aside from making
C.I.C. behave more reasonably, this is necessary infrastructure for some
aspects of the HOT patch.  Pavan Deolasee, with a little further stuff from
me.