remove session_manager_subscribe_xxx() API; convert session closing events to pseudo packets

when a session is closed, use the packet manager to create a pseudo packet,
    set the session to be closed as packet Exdata, and schedule it to the packet forwarding stage.
    when the pseudo packet free, the session will be free.

infra/tcp_reassembly/tcp_reassembly.h

@@ -12,7 +12,6 @@ struct tcp_segment
 {
      uint32_t len;
      const void *data;
-     void *user_data;
 };
 
 struct tcp_segment *tcp_segment_new(uint32_t seq, const void *data, uint32_t len);