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 <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Reviewed-by: Nazir Bilal Yavuz <[email protected]>
Discussion: 
https://postgr.es/m/CABPTF7VrqfbcDXqGrdLQ2xaQ=k0rzexnuw6u_ggqzsju32w...@mail.gmail.com

Branch
------
master

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

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

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