identification: Set possible answers to same msgid

Hello Julien,

I'm currently in the process of migrating to wendelin.core v2 on our WWM/wind clone. In the process of the migration I realized that NEO/go client currently doesn't support multiple master nodes. So I'm working on implementing this feature to NEO/go, so that we can finally use WCFS in our project.

There are some minor issues in NEO/go, which are quite straightforward to solve for me.

But what's difficult is to handle connecting to a master node if this master replies with a NotPrimaryMaster packet. It's simply difficult because the message id differs and the packet isn't proceeded in the same thread which expects the answer. The fastest-to-implement and simplest solution for this is to teach NEO/py to reply NotPrimaryMaster with the same msgid as the AcceptIdentification request. Because there are already some notifications which reply with the same msgid (replication AFAIK) this change shall be ok.

I set up a local test environment and run unit tests, functional tests and ZODB tests after I applied the given changes. No regressions appeared in any test:

unit tests:
	277 Tests, 0 Failed

zodb tests:
	109 Tests, 0 Failed

functional tests:
         29 Tests, 0 Failed

I'm not sure if the change in client/handlers/master.py is really necessary e.g. if it's really the same connection in _ask.

Best, Levin

/cc @klaus @jm