Made LLMediaDataClient not send requests on behalf of objects that are marked as dead.

When LLMediaDataClient::QueueTimer::tick() encounters an object at the head of the queue that's dead, it will now remove that object and loop, instead of sending a request and waiting for the tick timer to fire again.

Added an isDead() function to LLMediaDataClientObject, and an additional unit test that verifies the handling of dead objects.