refactor nbtsort-xx.c

[yanmeisun]

In issues #106, nbtsort-xx.c was suggested to be refactored as follow:

• If there are functions for only pg_bulkload in "lib/nbtree/nbtsort-XX.c", move the functions to "lib/nbtree/nbtsort-common.c"
• Copy PostgreSQL's latest stable "src/backend/access/nbtree/nbtsort.c" to "lib/nbtree/nbtsort-XX.c"

This PR does it. However PostgreSQL version under 9.5 has been EOL, refactoring wasn't done between nbtsort-8.3.c ~ nbtsort-9.5.c.

[mikecaat]

Hi,

Thanks for your works.

Copy PostgreSQL's latest stable "src/backend/access/nbtree/nbtsort.c" to "lib/nbtree/nbtsort-XX.c"

On the second thought, are just to remove nbtsort-XX.cc and to include "access/nbtree.h" enough?
To copy is redundant and it seems to have no advantage. If we use include "access/nbtree.h",
I think there is an advantage that we don't need to care minor (and major?) version up.

Sorry for changing my first suggestion. What do you think?