pgsql: postgres_fdw: Replace WAIT_EVENT_EXTENSION with custom wait even

postgres_fdw: Replace WAIT_EVENT_EXTENSION with custom wait events

Three custom wait events are added here:
- "PostgresFdwCleanupResult", waiting while cleaning up PQgetResult() on
transaction abort.
- "PostgresFdwConnect", waiting to establish a connection to a remote
server.
- "PostgresFdwGetResult", waiting to receive a result from a remote
server.

Author: Masahiro Ikeda
Discussion: https://postgr.es/m/197bce267fa691a0ac62c86c4ab904c4@oss.nttdata.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d61f2538a39f1dbeead01bc972fca597c769f518

Modified Files
--------------
contrib/postgres_fdw/connection.c | 23 ++++++++++++++++++++---
doc/src/sgml/postgres-fdw.sgml    | 38 ++++++++++++++++++++++++++++++++++++++
2 files changed, 58 insertions(+), 3 deletions(-)