Refactoring sign storage (on disk and in memory)

[tpwalke2]

All signs are currently stored in a single file and are loaded in memory.

This issue will cover a refactoring effort to use a more effective method of storing signs on disk and accessing them through the in-memory cache.

Some possibilities to consider:

• sqlite database
• storing signs by region
• storing signs by player
• storing signs by chunk (in memory)

[Janne252]

Thanks for providing this excellent plugin!

Assuming file based storage will be supported in the future, the sign storage file path probably should be moved from <server root>/config/bluemapsignmarkers/world/signs.json to <server root>/bluemapsignmarkers/world/signs.json so that the config directory isn't used for storing live data.