Store placeholders for offline players

[Dantevg]

Since some PlaceholderAPI plugins do not return placeholder data for offline players, store the data for them when they are online so we can use that stored data when they're offline.

• Storage method: database
• When server starts: load all data (to reduce IO)
• When player leaves: save their data
• When server closes: save all data

(came from #5 (comment))

[Dantevg]

Closed in 85e290e