Only isolate state when using reentrancy (#723)

Non-reentrant actors can utilize their state manager's cache
for repeated calls. They should still be able to do this unless
using reentrancy.

Co-authored-by: Ryan Nowak <[email protected]>

Author: halspang

src/Dapr.Actors/Runtime/ActorManager.cs

@@ -339,10 +339,13 @@ private async Task<T> DispatchInternalAsync<T>(ActorId actorId, ActorMethodConte
             T retval;
             try
             {
-                // Set the state context of the request, if possible.
-                if (state.Actor.StateManager is IActorContextualState contextualStateManager)
+                // Set the state context of the request, if required and possible.
+                if (ActorReentrancyContextAccessor.ReentrancyContext != null)
                 {
-                    await contextualStateManager.SetStateContext(Guid.NewGuid().ToString());
+                    if (state.Actor.StateManager is IActorContextualState contextualStateManager)
+                    {
+                        await contextualStateManager.SetStateContext(Guid.NewGuid().ToString());
+                    }
                 }
 
                 // invoke the function of the actor