Merge wal_level "archive" and "hot_standby" into new name "replica"

The distinction between "archive" and "hot_standby" existed only because
at the time "hot_standby" was added, there was some uncertainty about
stability.  This is now a long time ago.  We would like to move forward
with simplifying the replication configuration, but this distinction is
in the way, because a primary server cannot tell (without asking a
standby or predicting the future) which one of these would be the
appropriate level.

Pick a new name for the combined setting to make it clearer that it
covers all (non-logical) backup and replication uses.  The old values
are still accepted but are converted internally.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
Reviewed-by: David Steele <david@pgmasters.net>

doc/src/sgml/ref/alter_system.sgml

@@ -108,7 +108,7 @@ ALTER SYSTEM RESET ALL
   <para>
    Set the <literal>wal_level</>:
 <programlisting>
-ALTER SYSTEM SET wal_level = hot_standby;
+ALTER SYSTEM SET wal_level = replica;
 </programlisting>
   </para>