Only construct specs::Dispatcher once

* use version of shred that has an added SendDispatcher so we can
  construct the dispatcher and send it between threads (only State to
  remain sendable)
* move closure for adding systems from State::tick to the creation
  functions
* this does mean some voxygen systems always run instead of just in the
  session state, but that should not cause issues and we can always
  configure them to do nothing if needed