We could use browser.runtime.connect instead of browser.runtime.sendMessage to complete message delivery in xhr api.

The advantage of browser.runtime.connect is that it can maintain a long-lived connection between the object instance of the content scripts function and object instance of the background function. Since the message channel of the custom port name is exclusive, we no longer have to distinguish the source of the message, and the background will no longer need the xhrs global variable, all messages will be passed directly from object to object.

Ref: #315