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/SpecialMove.cs

@@ -290,8 +290,7 @@ namespace Server.Spells
             }
         }
 
-        private static SpecialMoveContext GetContext(Mobile m) =>
-            _playersTable.TryGetValue(m, out var context) ? context : null;
+        private static SpecialMoveContext GetContext(Mobile m) => _playersTable.GetValueOrDefault(m);
 
         private class SpecialMoveTimer : Timer
         {