Separate HEAP WAL replay logic into its own file

Hi PostgreSQL hackers,

For most access methods in PostgreSQL, the implementation of the access method itself and the implementation of its WAL replay logic are organized in separate source files. However, the HEAP access method is an exception. Both the access method and the WAL replay logic are collocated in the same heapam.c. To follow the pattern established by other access methods and to improve maintainability, I made the enclosed patch to separate HEAP’s replay logic into its own file. The changes are straightforward. Move the replay related functions into the new heapam_xlog.c file, push the common heap_execute_freeze_tuple() helper function into the heapam.h header, and adjust the build files.

I hope people find this straightforward refactoring helpful.

Yong