pgsql: Add read_stream_{pause,resume}()

Add read_stream_{pause,resume}()

Read stream users can now pause lookahead when no blocks are currently
available. After resuming, subsequent read_stream_next_buffer() calls
continue lookahead with the previous lookahead distance.

This is especially useful for read stream users with self-referential
access patterns (where consuming already-read buffers can produce
additional block numbers).

Author: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Reviewed-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA%2BhUKGJLT2JvWLEiBXMbkSSc5so_Y7%3DN%2BS2ce7npjLw8QL3d5w%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/38229cb905165fe676ab8728cb9dd0833a5534ed

Modified Files
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src/backend/storage/aio/read_stream.c | 27 +++++++++++++++++++++++++++
src/include/storage/read_stream.h | 2 ++
2 files changed, 29 insertions(+)