pgstattuple: Optimize pgstattuple_approx() with streaming read

pgstattuple: Optimize pgstattuple_approx() with streaming read

This commit plugs into pgstattuple_approx(), the SQL function faster
than pgstattuple() that returns approximate results, the streaming read
APIs.  A callback is used to be able to skip all-visible pages via VM
lookup, to match with the logic prior to this commit.

Under test conditions similar to 6c228755add8 (some dm_delay and
debug_io_direct=data), this can substantially improve the execution time
of the function, particularly for large relations.

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