Revamped buff system + skill cd/buff schedule rework

Completely rearranged buff system (system can smartly check for the best
statup to take effect for it's duration, or the server can be flagged to
act as usual). Refactored scheduling system for buffs expiration and
cooldowns to use one single thread per player rather than one per
instance.

src/client/MapleBuffStat.java

@@ -48,7 +48,7 @@ public enum MapleBuffStat {
     AURA(0x40000L),
     CONFUSE(0x80000L),
     
-    // ---- COUPON feature (was unused anyway) ----
+    // ------ COUPON feature ------
     
     COUPON_EXP1(0x100000L),
     COUPON_EXP2(0x200000L),