3413: added TrashHandler

* added a trashq WorkQueue to manage the trash list
* added TrashHandler to process "PUT /trash" requests
* added TestTrashHandler
Note: as of now the design doc is on a private wiki. If we're going to reference it from open source code, we should move it to the public wiki. I'm happy to do that, but let's review quickly whether there's anything sensitive that we should leave out.