savegame for testing: overall design, implement framework

[jdmonin]

Goal: To help with testing, the server can save a game and board's state to a file and load it later, using debug commands.

This ticket covers the overall design, and the rough draft of a framework to be fully implemented by other tickets.

Feature branch: feat/savegame

Usage/UI might be something like:

• Set value of a server property to point to the game-saves directory
• Start a game, place pieces as needed, etc
• Debug command to save a snapshot: *SAVEGAME* mygamename
• Debug command to load a snapshot: *LOADGAME* mygamename
  Server can parse the snapshot and create a game with its contents. Bots can be asked to join. (Maybe optionally require certain types of bots?) Temporarily set gamestate to a new hold/pause state, so current player won't take action until everyone has joined
• Debug command to resume play of loaded game: Maybe *RESUMEGAME* or *CONTINUEGAME*

The saved-game snapshot file:

Snapshot content would include game options, gameState, board layout, minimum version, and each player's pieces, dev cards, resource counts, etc. Players' names should probably also be included. Might want to save bot classes and smarter/faster flag or other parameters, but optionally not enforce as constraints when resuming play.

A transparent format like JSON probably makes the most sense (future-proofing, hand-editing, possibly later adding to a repo as test cases). One commonly-used JSON library is GSON, which is already used by the v3 branch by protobuf-java-util.

A dedicated savegame model class probably makes more sense than trying to serialize SOCBoard or SOCGame. Might be able to extract some board data structures by leveraging SOCGameHandler.getBoardLayoutMessage or similar existing joingame/startgame code.

Misc notes:

• To simplify things and prevent corner cases, don't allow save during initial placement
• Server config prop must designate a directory
• Optional json-library jar (GSON?) must be on classpath or same dir as server
• If json jar can't be loaded, server should degrade gently and run normally, as if the prop isn't set; savegames won't be used
• Use a default filename extension/suffix; should probably limit save-game filename's allowed characters for security
• Currently, debug commands can be typed only into a game window, but *LOADGAME* creates and joins a new game. At some point later, might want a better place in the UI to do so

[jdmonin]

Design validated, framework and basic load/save committed and tested (see #66).

[jdmonin]

Load/save/resume (#66) has been documented, implemented, tested, localized.