Display number of heap accesses for index scans

Hi,

right now it's hard to figure out whether a plain indexscan returns
matches that then get eliminated due to visibility checks in the
heap. For both index only scans (via "Heap Fetches") and bitmapscans
(via row mismatches between bitmap heap and index scans) one can gather
that to some degree from explain analyze.

The number of index lookups that failed to return anything can be a
critical performance factor in OLTP workloads. Therefore it seems like
it'd be a good idea to extend the explain analyze output to include that
information.

Greetings,

Andres Freund