pgsql: Rationalize GetWalRcv{Write,Flush}RecPtr().

Rationalize GetWalRcv{Write,Flush}RecPtr().

GetWalRcvWriteRecPtr() previously reported the latest *flushed*
location.  Adopt the conventional terminology used elsewhere in the tree
by renaming it to GetWalRcvFlushRecPtr(), and likewise for some related
variables that used the term "received".

Add a new definition of GetWalRcvWriteRecPtr(), which returns the latest
*written* value.  This will allow later patches to use the value for
non-data-integrity purposes, without having to wait for the flush
pointer to advance.

Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGJ4VJN8ttxScUFM8dOKX0BrBiboo5uz1cq%3DAovOddfHpA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d140f2f3e225ea53e2d92ab6833b8c186c90666c

Modified Files
--------------
src/backend/access/transam/xlog.c          | 20 ++++++++++----------
src/backend/access/transam/xlogfuncs.c     |  2 +-
src/backend/replication/README             |  2 +-
src/backend/replication/walreceiver.c      | 15 ++++++++++-----
src/backend/replication/walreceiverfuncs.c | 24 ++++++++++++++++++------
src/backend/replication/walsender.c        |  2 +-
src/include/replication/walreceiver.h      | 18 ++++++++++++++----
7 files changed, 55 insertions(+), 28 deletions(-)