Refactoring postgres_fdw code to rollback remote transaction

Hi,

In postgres_fdw, pgfdw_xact_callback() and pgfdw_subxact_callback() do
almost the same thing to rollback remote toplevel- and sub-transaction.
But their such rollback logics are implemented separately and
in different way. Which would decrease the readability and maintainability,
I think. So how about making the common function so that those callback
functions can just use it? Patch attached.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION