Fix bug where outstanding llHTTPRequests for scripts were not being aborted when they were deleted.

This was because AsyncCommandManager was handing an item ID to IHttpRequestModule.StopHttpRequest() rather than the expected request ID.
This commit also makes the http request asynchronous using BeginGetResponse() rather than doing this by launching a new thread
so that we can more safely abort it via HttpWebRequest.Abort() rather than aborting the thread itself.
This also renames StopHttpRequest() to StopHttpRequestsForScript() since any outstanding requests are now aborted and/or removed.