[Feature] Implement Configurable XHR Request Timeout in com.vaadin.client.communication.XhrConnection Vaadin 8

Current Issue

In Vaadin 8.11, our clients are experiencing intermittent issues where some UIDL XHR requests are taking ~5 minutes to resolve at random. Upon looking into configurable timeouts for XHR requests I found that this feature is currently a TODO in com.vaadin.client.communication.XhrConnection#send(JsonObject) in both v8.11 and v8.12:

    RequestBuilder rb = new RequestBuilder(RequestBuilder.POST, getUri());

    addXsrfHeaderFromCookie(rb);

    // TODO enable timeout
    // rb.setTimeoutMillis(timeoutMillis);
    // TODO this should be configurable
    rb.setHeader("Content-Type", JsonConstants.JSON_CONTENT_TYPE);
    rb.setRequestData(payload.toJson());

Expected behavior

A configurable value, whether this is done as a Servlet initial parameter, or through the UI instance, to set a timeout on XHR requests from the Client-side browser -> Vaadin Webserver.

Actual behavior

XHR Requests never timeout and if the response is never received the application is completely frozen until the user hard refreshes.

Start in com.vaadin.client.communication.XhrConnection#send(JsonObject), where the TODO and commented RequestBuilder timeout call are shown. Read the surrounding client communication code and determine how the timeout should be configured, since the issue lists servlet initialization and UI-instance options without choosing one. Done means browser XHR requests use the configured timeout instead of waiting indefinitely.