postgres_fdw: Refactor transaction rollback code to avoid code duplication.

postgres_fdw: Refactor transaction rollback code to avoid code duplication.

In postgres_fdw, pgfdw_xact_callback() and pgfdw_subxact_callback()
callback functions do almost the same thing to rollback remote toplevel-
and sub-transaction. But previously their such rollback logics were
implemented separately in each function and in different way. Which
could decrease the readability and maintainability of the code.

To fix the issue, this commit creates the common function to rollback
remote transactions, and makes those callback functions use it. Which
allows us to avoid unnecessary code duplication.

Author: Fujii Masao
Reviewed-by: Zhihong Yu, Bharath Rupireddy
Discussion: https://postgr.es/m/62fbb63a-d46c-fb47-a56d-f6be1909aa44@oss.nttdata.com