3413: added TrashHandler

* added a trashq WorkQueue to manage the trash list
* added TrashHandler to process "PUT /trash" requests
* added TestTrashHandler
In other words, build the `good_json` string dynamically? I feel like it's valuable to write the unit test to start with a hardcoded JSON string as input -- there's no possibility that the input we build is actually broken in a way that just happens to be compatible with the code that parses it. WDYT?