[Todo item] Add entry creation timestamp column to pg_stat_replication

Hello hackers,

I have worked on following todo list item.

- Add entry creation timestamp column to pg_stat_replication
http://archives.postgresql.org/pgsql-hackers/2011-08/msg00694.php

This item looks like simple because necessary data was already exist.
So, I wrote a prototype patch.

test example>
postgres=# select pid, reply_time from pg_stat_replication;
-[ RECORD 1 ]-----------------------------
pid | 4817
reply_time | 2018-07-31 12:00:53.911198+09
-[ RECORD 2 ]-----------------------------
pid | 4819
reply_time | 2018-07-31 12:00:53.911154+09


Several candidates exist for the field name.
- reply_timestamp
- info_gen_timestamp
- stats_reset
- last_msg_send_time

Feedback and suggestion will be very welcome.
Thanks!

Best regards,
Myungkyu, Lim