Python: request processing in multiple threads.

This closes #459 issue on GitHub.

src/nxt_application.h

@@ -51,6 +51,8 @@ typedef struct {
     nxt_str_t  path;
     nxt_str_t  module;
     char       *callable;
+    uint32_t   threads;
+    uint32_t   thread_stack_size;
 } nxt_python_app_conf_t;