pgsql: 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(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
Discussion: https://postgr.es/m/CABPTF7VrqfbcDXqGrdLQ2xaQ=K0RzExNuw6U_GGqzSJu32wfdQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ae58189a4d523f0156ebe30f4534180555669e88

Modified Files
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contrib/pgstattuple/pgstatapprox.c | 127 ++++++++++++++++++++++++++++---------
src/tools/pgindent/typedefs.list | 1 +
2 files changed, 97 insertions(+), 31 deletions(-)