Change .Promary type from int8 back to int32.

5d93e434 says that .Primary type is not NodeID. That is true, but
changing it to int8 was a mistake:

1. PSignedNull is explicitly defined to come with '!l' struct code, which
   according to https://docs.python.org/3/library/struct.html#module-struct
   comes as 4-bytes integer on the wire:

   https://lab.nexedi.com/nexedi/neoppod/blob/v1.12-13-gf2ea4be2/neo/lib/protocol.py#L560-562

2. verifying this via serializing NotPrimaryMaster on NEO/py also
   confirms that .Primary occupies 4 bytes, not one:

   In [1]: from neo.lib.protocol import NotPrimaryMaster

   In [2]: NotPrimaryMaster(0x01020304, [('m111', 111), ('m222', 222)])._body
   Out[2]: '\x01\x02\x03\x04\x00\x00\x00\x02\x00\x00\x00\x04m111\x00o\x00\x00\x00\x04m222\x00\xde'
            ^^^^^^^^^^^^^^^^
            NOTE  NOTE  NOTE

-> change .Primary type back to being 4-bytes integer, but to int32
instead of NodeID because, as 5d93e434 correctly says, .Primary comes as
array index, not a node ID. The following place of NEO/py code
explicitly confirms this:

https://lab.nexedi.com/nexedi/neoppod/blob/v1.12-13-gf2ea4be2/neo/master/handlers/identification.py#L155-159

Add corresponding test.