pgsql: Move commonly used context into PruneState and simplify helpers

Move commonly used context into PruneState and simplify helpers

heap_page_prune_and_freeze() and many of its helpers use the heap
buffer, block number, and page. Other helpers took the heap page and
didn't use it. Initializing these values once during
prune_freeze_setup() simplifies the helpers' interfaces and avoids any
repeated calls to BufferGetBlockNumber() and BufferGetPage().

While updating PruneState, also reorganize its fields to make the layout
and member documentation more consistent.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/BD8B69E7-26D8-4706-9164-597C6AE57812%40gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/59663e4207fdbf8ffffad0c378f0c455ce2beb06

Modified Files
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src/backend/access/heap/pruneheap.c | 136 +++++++++++++++++++-----------------
1 file changed, 72 insertions(+), 64 deletions(-)