tor-dirmgr/state.rs: add new NetDirChange API, consume it

- The new DirState::get_netdir_change() API lets the state machine
  export a NetDirChange: a request to either replace the current netdir,
  or add microdescs to it.
- bootstrap.rs now consumes this new API, even though nothing implements
  it yet.
- This will let us implement GetMicrodescsState without having to
  directly mutate the netdir. The calling code also handles checking the
  netdir against the circmgr for sufficiency, and updating the consensus
  metadata in the store, meaning the revised GetMicrodescsState will not
  have to perform these tasks.