replace wake with wake_later for User_buffer.Rx

- before this change, http-based get_state takes an inconsistent
  snapshot, since many threads that will take values from their Lwt.mvar
  are delayed while get_state thread is immediately woken up to take a
  snapshot. wake_later ensures that snapshot thread is running after all
  taking threads complete to run.

- this version can do get/set snapshot on the same host in the middle of
  flow.