Emote cache

[Vinrobot]

Use cache to not download already loaded emote.

• Apache HttpClient Cache with Ehcache
• Apache HttpClient Cache with custom cache
• Custom Java HttpClient with cache (https://github.com/ben-manes/caffeine)
• Something else?

Also need to check if Minecraft use something internally.

[Vinrobot]

Basic implementation in 1024cbf .
One request per file, no index, no maximum cache size.

TODO:

• 4-steps write (Write "tmp", move existing file to "bak", commit temp, remove "bak")
• Maximum cache size (+ settings)
• Clearable in settings