Added the ability to optionally specify the visibility timeout when d…

…equeuing a message from an Azure Storage Queue

The existing Dequeue member  of the ProvidedQueue type gains an
additional optional Timespan parameter. I also added an overload that
accepts a TimeSpan and uses the default connection string. The net
effect is to allow the user to call Dequeue in the following ways:

Dequeue() -> Dequeue(?connectionString, ?visibilityTimeout)
Dequeue("some connection string") -> Dequeue(?connectionString,
?visibilityTimeout)
Dequeue("some connection string",  TimeSpan.FromMinutes(1..0) ->
Dequeue(?connectionString, ?visibilityTimeout)
Dequeue(TimeSpan.FromMinutes(1.0) -> Dequeue(visibilityTimeout)

src/FSharp.Azure.StorageTypeProvider/Queue/ProvidedQueueTypes.fs

@@ -72,16 +72,26 @@ type ProvidedQueue(defaultConnectionString, name) =
         if queueRef.ApproximateMessageCount.HasValue then queueRef.ApproximateMessageCount.Value
         else 0
 
-    /// Dequeues the next message.
-    member __.Dequeue(?connectionString) = 
+    /// Dequeues the next message and optionally sets the visibilityTimeout (i.e. how long you can work with the message before it reappears in the queue)
+    member __.Dequeue(?connectionString, ?visibilityTimeout) = 
         async { 
-            let! message = (getQueue connectionString).GetMessageAsync() |> Async.AwaitTask
+            let! message = (getQueue connectionString).GetMessageAsync(visibilityTimeout |> Option.toNullable, null, null) |> Async.AwaitTask
             return
                 match message with
                 | null -> None
                 | _ -> Some(message |> Factory.toProvidedQueueMessage)
         }
-
+
+    /// Dequeues the next message using the default connection string and sets the visibilityTimeout (i.e. how long you can work with the message before it reappears in the queue)
+    member __.Dequeue(visibilityTimeout) = 
+        async { 
+            let! message = (getQueueRef name defaultConnectionString).GetMessageAsync(visibilityTimeout |> Nullable , null, null) |> Async.AwaitTask
+            return
+                match message with
+                | null -> None
+                | _ -> Some(message |> Factory.toProvidedQueueMessage)
+        }
+
     /// Generates a full-access shared access signature, defaulting to start from now.
     member __.GenerateSharedAccessSignature(duration, ?start, ?connectionString) = 
         getQueue connectionString |> generateSas start duration