Remove unnecessary unbind in LDAP search+bind mode

Comments in src/backend/libpq/auth.c say: (after successfully finding
the final DN to check the user-supplied password against)

/* Unbind and disconnect from the LDAP server */

and later

/*
 * Need to re-initialize the LDAP connection, so that we can bind to
 * it with a different username.
 */

But the protocol actually permits multiple subsequent authentications
("binds") over a single connection.

So, it seems like the whole connection re-initialization thing was
just a confusion and can be safely removed, thus saving quite a few
network round-trips, especially for the case of ldaps/starttls.

Author: Anatoly Zaretsky <anatoly.zaretsky@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CALbq6kmJ-1+58df4B51ctPfTOSyPbY8Qi2=ct8oR=i4TamkUoQ@mail.gmail.com

Author: petere

doc/src/sgml/client-auth.sgml

@@ -1775,13 +1775,13 @@ omicron         bryanh                  guest1
     do an exact match of the attribute specified in
     <replaceable>ldapsearchattribute</replaceable>.
     Once the user has been found in
-    this search, the server disconnects and re-binds to the directory as
+    this search, the server re-binds to the directory as
     this user, using the password specified by the client, to verify that the
     login is correct. This mode is the same as that used by LDAP authentication
     schemes in other software, such as Apache <literal>mod_authnz_ldap</literal> and <literal>pam_ldap</literal>.
     This method allows for significantly more flexibility
     in where the user objects are located in the directory, but will cause
-    two separate connections to the LDAP server to be made.
+    two additional requests to the LDAP server to be made.
    </para>
 
    <para>
@@ -2008,7 +2008,7 @@ host ... ldap ldapserver=ldap.example.net ldapbasedn="dc=example, dc=net" ldapse
     the LDAP server, perform a search for <literal>(uid=someuser)</literal>
     under the specified base DN.  If an entry is found, it will then attempt to
     bind using that found information and the password supplied by the client.
-    If that second connection succeeds, the database access is granted.
+    If that second bind succeeds, the database access is granted.
    </para>
 
    <para>

src/backend/libpq/auth.c

@@ -2611,31 +2611,6 @@ CheckLDAPAuth(Port *port)
 		pfree(filter);
 		ldap_memfree(dn);
 		ldap_msgfree(search_message);
-
-		/* Unbind and disconnect from the LDAP server */
-		r = ldap_unbind_s(ldap);
-		if (r != LDAP_SUCCESS)
-		{
-			ereport(LOG,
-					(errmsg("could not unbind after searching for user \"%s\" on server \"%s\"",
-							fulluser, server_name)));
-			pfree(passwd);
-			pfree(fulluser);
-			return STATUS_ERROR;
-		}
-
-		/*
-		 * Need to re-initialize the LDAP connection, so that we can bind to
-		 * it with a different username.
-		 */
-		if (InitializeLDAPConnection(port, &ldap) == STATUS_ERROR)
-		{
-			pfree(passwd);
-			pfree(fulluser);
-
-			/* Error message already sent */
-			return STATUS_ERROR;
-		}
 	}
 	else
 		fulluser = psprintf("%s%s%s",