Add payload base classes to prepare for simplifying the client

This change adds payload request and response base classes to
prepare for future simplification updates to the current client
architecture. No new tests are required for this change.

kmip/core/messages/payloads/poll.py

@@ -18,9 +18,10 @@ import six
 from kmip import enums
 from kmip.core import primitives
 from kmip.core import utils
+from kmip.core.messages.payloads import base
 
 
-class PollRequestPayload(primitives.Struct):
+class PollRequestPayload(base.RequestPayload):
     """
     A request payload for the Poll operation.
 
@@ -38,9 +39,7 @@ class PollRequestPayload(primitives.Struct):
                 operation to poll the status of, in bytes. Optional, defaults
                 to None.
         """
-        super(PollRequestPayload, self).__init__(
-            enums.Tags.REQUEST_PAYLOAD
-        )
+        super(PollRequestPayload, self).__init__()
 
         self._asynchronous_correlation_value = None
         self.asynchronous_correlation_value = asynchronous_correlation_value