PP request cancelled without notification

[schloett]

When subsequent requests are sent from one tab via the background script to the PP, requests, which did not finish until the arrival of a new request are canceled. However, the canceling applies to all tabs, i..e. when tab1 sends a request and tab2 sends a request afterwards, the request of tab1 will be canceled if not finished yet. Tab1 is not informed about this cancellation and the loading indicator will spin forever.