Reload points cache off the main thread

[GG-MD]

The Guava points cache uses refreshAfterWrite, but Guava's default reload runs
synchronously on the calling thread. So every ~30s a give/take on an
online player triggers a blocking SQLite query on the main server thread - spark
showed ~276ms stalls parked on the SQLite connection monitor.

Wrapped the CacheLoader in CacheLoader.asyncReloading(...), running refreshes
on the plugin's async scheduler instead. Stale entries are returned immediately
and refreshed in the background - no more tick stalls.

Balance correctness is unaffected: Guava drops the stale reload result if a
put landed while it was running, and pending transactions are layered on top
in getEffectivePoints() regardless.

Note: this only covers refresh of already-cached entries. A cold cache
miss (offline player, or an entry evicted after 5 min) still loads synchronously

• same as before, out of scope here.

[GG-MD]

@Esophose