When clearing entries, set expiry time to now. Addresses #786

This is so that the allocate_find will return the oldest IP.
And, the allocate_find will now use an index, rather than
potentially scanning the entire table

raddb/mods-config/sql/ippool-dhcp/mysql/queries.conf

@@ -30,7 +30,7 @@ allocate_clear = "\
 		pool_key = 0, \
 		callingstationid = '', \
 		username = '', \
-		expiry_time = NULL \
+		expiry_time = NOW() \
 	WHERE expiry_time <= NOW() - INTERVAL 1 SECOND \
 	AND nasipaddress = '%{Nas-IP-Address}'"
 
@@ -64,7 +64,7 @@ allocate_find = "\
 #	SELECT framedipaddress \
 #	FROM ${ippool_table} \
 #	WHERE pool_name = '%{control:Pool-Name}' \
-#	AND expiry_time IS NULL \
+#	AND expiry_time < NOW() \
 #	ORDER BY RAND() \
 #	LIMIT 1 \
 #	FOR UPDATE"