Fixed a thread safety issue in BsonClassMap.

Author: rstam

src/MongoDB.Bson/Serialization/BsonClassMap.cs

@@ -295,7 +295,15 @@ public static Type GetMemberInfoType(MemberInfo memberInfo)
         /// <returns>All registered class maps.</returns>
         public static IEnumerable<BsonClassMap> GetRegisteredClassMaps()
         {
-            return __classMaps.Values;
+            BsonSerializer.ConfigLock.EnterReadLock();
+            try
+            {
+                return __classMaps.Values.ToList(); // return a copy for thread safety
+            }
+            finally
+            {
+                BsonSerializer.ConfigLock.ExitReadLock();
+            }
         }
 
         /// <summary>