pgstattuple: Optimize btree and hash index functions with streaming read

pgstattuple: Optimize btree and hash index functions with streaming read

This commit replaces the synchronous ReadBufferExtended() loops with the
streaming read routines, affecting pgstatindex() (for btree) and
pgstathashindex() (for hash indexes).

Under test conditions similar to 6c228755add8 (some dm_delay and
debug_io_direct=data), this can result in nice runtime and IO gains.

Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CABPTF7VrqfbcDXqGrdLQ2xaQ=K0RzExNuw6U_GGqzSJu32wfdQ@mail.gmail.com