Docs: Update the protocol to provide mechanisms for ClientAgentAddInterest.
Unittests: Add a unittest for ClientAgent(Add|Remove)Interest(Multiple)?
ClientAgent: Implement server-side interest opening and closing.
Unittests: Add unittest for Declare and Undeclare object in ClientAgent.
ClientAgent: Implement declare and undeclare object.
Docs: Remove CLIENT_OBJECT_SET_FIELDS from Client protocol.

This feature is unimplemented, improperly documented, untested, and as
far as I can tell, not used or wanted by libastron.  If someone later
finds a use for this message, we can re-add it.
Unittests: Write unittest for set_fields_sendable.
ClientAgent: Implement set_fields_sendable.
Unittests: Mitigate race condition in unittest.
Docs: Add two new client disconnect codes for interest config.
Unittest: Add unittest for disable client-side interest adding.
ClientAgent: Implement config for client-side interest adding.
Unittests: Update unittests to test out the "visible"-only config setting for interests.
ClientAgent: Remove extra space from log message in AstronClient.cpp.
Docs: Update protocol to specify handling of corner-cases for Session objects.
Unittests: Flush at the beginning of each clientAgent unittests so failing tests don't break eachother.
Docs: Fix mix typos/mistakes in protocol text.

Fixed:
 - Changing owner should say it sends a changing owner message.
 - Session objects must be visible or owned, and can't be declared.
Tests: Add ClientAgent unittest for adding/removing session objects.
ClientAgent: Implement adding/removing session objects.
Unittests: Add unittest for loss of ownership after CHANGING_OWNER.