fix: Fixes various memory leaks and spells (#1786)

### Summary
- Fixes various memory leaks related to spells
- Fixes Spell Plague
- Added ability to determine if `sdi` should take effect for Spell Damage.
- Fixes animal form timer ticking non-stop while logged out.

Projects/UOContent/Spells/Base/SpellHelper.cs

@@ -1022,6 +1022,7 @@ namespace Server.Spells
                 StaminaSystem.DFA = dfa;
 
                 var damageGiven = AOS.Damage(target, from, dmg, phys, fire, cold, pois, nrgy, chaos);
+                Mysticism.SpellPlagueSpell.OnMobileDamaged(target);
 
                 StaminaSystem.DFA = DFAlgorithm.Standard;