I2PTunnel: Call destroySocketManager() when replacing

  a client socket manager configured with i2cp.newDestOnResume,
  or a shared client.
  This prevents lots of orphaned StreamingTimer threads.
  There are still cases where these threads may stick around,
  when a tunnel is stopped but not restarted. We don't know in
  some cases when a tunnel could be restarted and when it can't.
  destroySocketManager() should always be called when discarding a
  SocketManager but i2ptunnel doesn't do that now. More to do,
  but this fixes the most common case of leaked threads.
  Related tickets: ##642 #961