Provide optimum access method performance

To provide the optimum performance with an access method, perform the following actions:

• Provide am_scancost and am_stats purpose functions.
• Split scan processing into am_beginscan, am_getnext, am_rescan, and am_endscan purpose functions.
• Return more than one row from am_getnext or am_rescan, as Buffering multiple results describes.
• Register purpose functions as parallelizable, as Executing in parallel describes.