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.