pg_stat_wal_receiver and flushedUpto/writtenUpto

Hi,

As discussed in the thread that introduced d140f2f3 to rename
receivedUpto to flushedUpto and add writtenUpto to the WAL receiver's
shared memory information, the equivalent columns in
pg_stat_wal_receiver have not been renamed:
https://www.postgresql.org/message-id/CA+hUKGJ06d3h5JeOtAv4h52n0vG1jOPZxqMCn5FySJQUVZA32w@mail.gmail.com

When I have implemented this system view, the idea was to keep a
one-one mapping between the SQL interface and the shmem info even if
we are not compatible with past versions, hence I think that before
beta1 we had better fix that and:
- rename received_lsn to flushed_lsn.
- add one column for writtenUpto.

Attached is a patch to do that.  This needs also a catalog version
bump, and I am adding an open item.
Thanks,
--
Michael