identification: Set possible answers to same msgid

When a node tries to connect to another node it initially sends a
'RequestIdentification' packet. The other node can either reply with
'AcceptIdentification' or in case of a secondary master with
'NotPrimaryMaster'.

In the second case the message id differs from the initial requests
message id. This makes it difficult in a multi-threaded implementation
to proceed this answer: due to the different msg/connection - id the
multi-threaded implementation tries to proceed this incoming message in
a different thread, while the requesting thread waits forever for its
peers reply.

The most straightforward solution is to use the same connection - id for
both possible answers to the 'RequestIdentification' packet. This
doesn't break given NEO/py implementation and is only a small patch.
A workaround in a multi-threaded implementation on the other hand
seems to be much more complicated and time-consuming. Finally it also makes
sense semantically, because "Message IDs are used to identify response
packets" and in the given context the 'NotPrimaryMaster' *is* the de facto
response of a 'RequestIdentification'.

/suggested-at !2
/proposed-for-review-at nexedi/neoppod!20

neo/master/handlers/identification.py

@@ -156,5 +156,5 @@ class SecondaryIdentificationHandler(EventHandler):
             for node in app.nm.getMasterList()]
         conn.send(Packets.NotPrimaryMaster(
             primary and known_master_list.index(primary),
-            known_master_list))
+            known_master_list), msg_id=conn.peer_id)
         conn.abort()