Correction for replication slot creation error message in 9.6

Hi

Currently pg_create_physical_replication_slot() may refer to
the deprecated wal_level setting "archive":

  postgres=# SHOW wal_level ;
   wal_level
  -----------
   minimal
  (1 row)

  postgres=# SELECT pg_create_physical_replication_slot('some_slot');
  ERROR:  replication slots can only be used if wal_level is  >= archive

Patch changes the error message to:

  ERROR:  replication slots can only be used if wal_level is "replica" or "logical"

Explicitly naming the valid WAL levels matches the wording in the wal_level
error hint used in a couple of places, i.e.

  "wal_level must be set to "replica" or "logical" at server start."


Regards

Ian Barwick

--
 Ian Barwick                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services