on the main thread.

 <p>
     The flow for any request is as follows:
     <ul>
         <li>Check the memory cache and provide the cached resource if present</li>
         <li>Check the current set of actively used resources and return the active resource if present</li>
         <li>Check the current set of in progress loads and add the cb to the in progress load if present</li>
         <li>Start a new load</li>
     </ul>
 </p>

 <p>
     Active resources are those that have been provided to at least o