Unix sockets support

[Furrrlo]

This PR adds support for using Unix sockets instead of the default TCP one, either via the junixsocket library (Java 1.7+) or the SocketChannel implementation introduced by Java 16.

It should completely mantain compatibility with the old API, it was mostly internal changes, with the only addition of 1 constructor to the Unique4j class. The only minor inconvenience is that I had to bump the Java version of the core module to 1.7, as the Java 16 compiler doesn't allow to go lower than that.

Example usage (p much the same):

// create unique instance
/* Unique4j unique = new Unique4j(APP_ID, true, new UnixSocketChannelIpcFactory()) { */
Unique4j unique = new Unique4j(APP_ID, true, new AFUNIXSocketIpcFactory()) {
    @Override
    public void receiveMessage(String message) {
        // print received message (timestamp)
        System.out.println(message);
    }

    @Override
    public String sendMessage() {
        // send timestamp as message
        Timestamp ts = new Timestamp(new Date().getTime());
        return "Another instance launch attempted: " + ts.toString();
    }
    
    /* It is not mandatory to override this method
     * By default, the stack trace is printed
     */
    @Override
    public void handleException(Exception exception) {
        // display the exception message
        System.out.println(exception.getMessage());
    }

    /* It is not mandatory to override this method
     * By default, the subsequent instance simply exits
     * This method is not invoked if AUTO_EXIT is turned off
     */
    @Override
    public void beforeExit() {
        // display exit message
        System.out.println("Exiting subsequent instance.");
    }
};

I had to move a bunch of stuff around and add a bit of scaffolding so the PR is kinda big, but I thought I would still try to contribute it back.