pgsql: Further refactor of heapgettup and heapgettup_pagemode

Further refactor of heapgettup and heapgettup_pagemode

Backward and forward scans share much of the same page acquisition code.
Here we consolidate that code to reduce some duplication.

Additionally, add a new rs_coffset field to HeapScanDescData to track the
offset of the current tuple.  The new field fits nicely into the padding
between a bool and BlockNumber field and saves having to look at the last
returned tuple to figure out which offset we should be looking at for the
current tuple.

Author: Melanie Plageman
Reviewed-by: David Rowley
Discussion: https://postgr.es/m/CAAKRu_bvkhka0CZQun28KTqhuUh5ZqY=_T8QEqZqOL02rpi2bw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f9bc34fcb664e93d1fa3c3873527dce6f4cc495f

Modified Files
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src/backend/access/heap/heapam.c | 200 ++++++++++++---------------------------
src/include/access/heapam.h      |   1 +
2 files changed, 63 insertions(+), 138 deletions(-)