Integrate policy file monitoring with the server

This change updates the server, integrating policy file monitoring
and restructuring the engine. The top-level server entity now
handles loading policy files using the PolicyDirectoryMonitor
subprocess. A shared memory dictionary is used to share newly
modified policy data across the session threads managed by the
server and used by the engine. The legacy policy loading code in
the engine has been removed.

Unit tests have been added and modified for both the server and
engine to verify the functionality of these modifications.

kmip/core/exceptions.py

@@ -311,6 +311,12 @@ class InvalidPrimitiveLength(Exception):
     pass
 
 
+class ShutdownError(Exception):
+    """
+    An error generated when a problem occurs with shutting down the server.
+    """
+
+
 class StreamNotEmptyError(Exception):
     def __init__(self, cls, extra):
         super(StreamNotEmptyError, self).__init__()