Add TID Range Scans to support efficient scanning ranges of TIDs

 Add TID Range Scans to support efficient scanning ranges of TIDs
 
 This adds a new executor node named TID Range Scan.  The query planner
 will generate paths for TID Range scans when quals are discovered on base
 relations which search for ranges on the table's ctid column.  These
 ranges may be open at either end. For example, WHERE ctid >= '(10,0)';
 will return all tuples on page 10 and over.
 
 To support this, two new optional callback functions have been added to
 table AM.  scan_set_tidrange is used to set the scan range to just the
 given range of TIDs.  scan_getnextslot_tidrange fetches the next tuple
 in the given range.
 
 For AMs were scanning ranges of TIDs would not make sense, these functions
 can be set to NULL in the TableAmRoutine.  The query planner won't
 generate TID Range Scan Paths in that case.
 
 Author: Edmund Horner, David Rowley
 Reviewed-by: David Rowley, Tomas Vondra, Tom Lane, Andres Freund, Zhihong Yu
 Discussion: https://postgr.es/m/CAMyN-kB-nFTkF=VA_JPwFNo08S0d-Yk0F741S2B7LDmYAi8eyA@mail.gmail.com