Add conninfo to pg_stat_wal_receiver

Commit b1a9bad9e7 introduced a stats view to provide insight into the
running WAL receiver, but neglected to include the connection string in
it, as reported by Michaël Paquier.  This commit fixes that omission.
(Any security-sensitive information is not disclosed).

While at it, close the mild security hole that we were exposing the
password in the connection string in shared memory.  This isn't
user-accessible, but it still looks like a good idea to avoid having the
cleartext password in memory.

Author: Michaël Paquier, Álvaro Herrera
Review by: Vik Fearing

Discussion: https://www.postgresql.org/message-id/CAB7nPqStg4M561obo7ryZ5G+fUydG4v1Ajs1xZT1ujtu+woRag@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -1302,6 +1302,14 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
      <entry><type>text</></entry>
      <entry>Replication slot name used by this WAL receiver</entry>
     </row>
+    <row>
+     <entry><structfield>conn_info</></entry>
+     <entry><type>text</></entry>
+     <entry>
+      Connection string used by this WAL receiver,
+      with security-sensitive fields obfuscated.
+     </entry>
+    </row>
    </tbody>
    </tgroup>
   </table>