Per query FDW network stat collection

Hello,

I have implemented per query network stat collection for FDW. It is done
in a similar way to how buffer and WAL stats are collected and it can be
seen with a new NETWORK option for explain command:

explain (analyze, network) insert into itrtest values (2, 'blah');

                                          QUERY PLAN
-----------------------------------------------------------------------------------------------
 Insert on itrtest  (cost=0.00..0.01 rows=0 width=0) (actual
time=0.544..0.544 rows=0 loops=1)
   Network: FDW bytes sent=197 received=72, wait_time=0.689
   ->  Result  (cost=0.00..0.01 rows=1 width=36) (actual
time=0.003..0.003 rows=1 loops=1)
 Planning Time: 0.025 ms
 Execution Time: 0.701 ms
(5 rows)

I am yet to add corresponding columns to pg_stat_statements, write tests
and documentation, but before I go ahead with that, I would like to know
what the community thinks about the patch.

Regards,

Ilya Gladyshev