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