chore(content): Cleans up abilities and some spell timers (#427)

- [X] Exposes Index/Count on the timer.
- [X] Cleans up abilities
- [X] Combines some spell context/info objects with their timers to reduce allocations
- [X] Fixes a bug where immolating weapon both finishes effect or stops in the wrong order due to a race condition.

Projects/UOContent/Spells/Base/Spell.cs

@@ -838,10 +838,9 @@ namespace Server.Spells
 
             public void Add(Mobile m, Timer t)
             {
-                if (m_Contexts.TryGetValue(m, out var oldTimer))
+                if (m_Contexts.Remove(m, out var oldTimer))
                 {
                     oldTimer.Stop();
-                    m_Contexts.Remove(m);
                 }
 
                 m_Contexts.Add(m, t);
@@ -850,6 +849,7 @@ namespace Server.Spells
             public void Remove(Mobile m)
             {
                 m_Contexts.Remove(m);
+                // TODO: Should we stop the timer?
             }
         }