Fix race in memory tracker (#254)

* fixes a data race in memory tracker that can cause divide-by-zero exceptions

* Add comment to clarify that there is a potential race condition

Co-authored-by: David Justo <david.justo.1996@gmail.com>

---------

Co-authored-by: David Justo <david.justo.1996@gmail.com>

src/DurableTask.Netherite/StorageLayer/Faster/MemoryTracker.cs

@@ -41,7 +41,9 @@ namespace DurableTask.Netherite.Faster
 
         public void UpdateTargetSizes()
         {
-            if (this.stores.Count > 0)
+            // store count to avoid race condition where count goes to zero and causes divide-by-zero error
+            int numberOfStores = this.stores.Count;
+            if (numberOfStores > 0)
             {
                 long targetSize = this.maxCacheSize / this.stores.Count;
                 foreach (var s in this.stores.Keys)