Refactoring nbtsort-xx.c for maintenability

[mikecaat]

pg_bulkload has codes related to nbtsort in "lib/nbtree/nbtsort-XX.c", which XX is each postgresql's version. And it has both postgresql's code and functions for only pg_bulkloads.

For maintenability, it's better to refactor "nbtsort-XX.c" and become "nbtsort-XX.c" to be just a copy of PostgreSQL's "src/backend/access/nbtree/nbtsort.c". So, this issue handles the followings.

• 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"

[yanmeisun]

Dear @mikecaat
I create a PR #110 to do it. Could you please review it?
By the way, since PostgreSQL version under 9.5 has been EOL, I am doing nothing to nbtsort-8.3.c ~ nbtsort-9.5.c.

[mikecaat]

Thanks a lot!

I agree that the under 9.6(?) has been EOL, so it's ok doing nothing to nbtsort-8.3.c ~ nbtsort-9.5.c.
I have a comment to the PR. Please check it.