Requests started before onStart may never run

[sjudd]

RequestTracker only maintains a map of weak references to requests. RequestTracker also does not immediately run requests if it believes the Fragment or activity is not started. Requests into non-view targets may not be strongly referenced outside of Glide's request system. As a result, requests into non-view targets made before onStart() may be only weakly referenced and be garbage collected before the fragment or activity is started, preventing the request from ever running.