Reuse Volley request queues instead of creating a new one each time

I don't see how using a static map here is so bad, other than the memory footprint. Each queue is associated with a ```SalesforceHttpStack```, which contains an auth token that uniquely associates it with a user. Reusing/sharing them across users is likely to be messy and breaks the existing model of one queue per user. As for the memory leak, I see only one case where we don't release the queue, and that is when a user logs out, which I'd imagine is a pretty rare occurrence in the wild. Also, if that same user were to log back in, the existing ```RequestQueue``` would be reused for that user.