Private group invitation protocol
This branch implements the private group invitation protocol. The implementation is something of an experiment with a new way of writing client protocols.
We start with a role enum that lists the roles in the protocol, and a state enum for each role, which lists the states in the role's state machine. Then there's a session class, parameterised by the state class and therefore by the role, which represents the session information held by that role. Then there's an engine interface, parameterised by the session class and therefore by the role, which encapsulates the protocol logic for the role. Most of this stuff can be created pretty mechanically from the state machine diagrams.
The engine interface has a method for each type of message and each local action. I started out with one method for all messages and another for all local actions, but that turned out to be a bad design - the information about what kind of message was being handled was lost when the message was passed to the engine, and had to be recovered using an instanceof ladder.
Each engine method takes a message or an action and a session, and returns an updated session. A transaction is passed in so the engine can send messages, attach events, and do any other work it needs to do (such as changing the visibility of groups, in the case of this protocol). This removes the need to run tasks outside the engine, so the protocol logic is better encapsulated inside the engine.
Parsing and encoding of messages and sessions is separated from protocol logic. MessageParser, MessageEncoder and the validator are the only classes that know how messages and their metadata are formatted, and likewise SessionParser and SessionEncoder are the only classes that know how sessions are formatted. The metadata keys are declared in a package-private interface.
It's common knowledge that I never make mistakes, so to keep things interesting I've hidden 114 deliberate mistakes in this code. See how many you can spot!
Needs tests before #709 (closed) is closed.