SoupSessionFeature: Keep features alive until all messages have been unqueued

Features can be removed at any point from a session, and if they are removed while there are messages in the queue, it is possible for callbacks connected to the message signals to be called after the feature has been disposed. Adding a reference in request_queue() and removing it in request_enqueue() ensures that the feature will not be disposed too early.

Add a test case for the SoupCookieJar feature that reproduces this crash.

Fixes #130 (closed)