2769: implement DELETE.

2769: implement DELETE.

The DeleteHandler method implements DELETE requests for Keep, checking
that the user is authorized to issue DELETE and then deleting the
requested block from local volumes.

* CanDelete checks that the user with the given token is authorized to
  delete blocks from Keep.
* MockVolume and UnixVolume objects provide a Delete method for deleting
  a block from that volume.
* TestDeleteHandler tests:
** Unauthenticated requests
** Requests from a non-admin user
** Requests from a non-admin user for a nonexistent block
** Requests from a non-admin user for an existing block