- Refactor to allow running a single Snark without a SnarkManager again,
    by moving some things from SnarkManager to I2PSnarkUtil,
    having Snark call completeListener callbacks,
    and having Storage call storageListener callbacks.
    This is in preparation for using Snark for router updates.
    Step 2 is to allow multiple I2PSnarkUtil instances.
  - Big rewrite of Storage to open file descriptors on demand, and
    close them when unused, so we can support large numbers of torrents.