The buildd slaves and the buildd manager should guard against protocol mismatches

As recently observed with recipe builds, a protocol change on the master side can effectively kill a buildd if it was not changed in tandem. (expected fields were missing in the request)

To counter this, we need to version the protocol between the slaves and the manager. The manager should send a protocol version with each request and the slave should reject the message if the version doesn't meet expectations.

We also need a way to have this version automatically bumped when the protocol does change, otherwise it's useless. Maybe some sort of hash of the field names instead of a version number?