pgstattuple: Use double consistently for percentages

pgstattuple: Use double consistently for percentages

pgstattuple uses data type double for other percentage calculations
and exposes those values to the users via the float8 data type.
However, scanned_percent in struct output_type is of type uint64, even
though it is later returned via Float8GetDatum().  Change it to use
double to be inline with other percentages.

Author: Bharath Rupireddy
Reviewed-by: Peter Eisentraut
Discussion: https://www.postgresql.org/message-id/36ee692b-232f-0484-ce94-dc39d82021ad%40enterprisedb.com