v0.5.7

Added

• ServerBuilder::environment_hook — an EnvironmentHook the server runs on
  the engine environment_open has just created, before that engine becomes
  the environment. The engine is new on every open, so anything a client
  installed on the last one (the beamline's suspenders, say) is gone, and the
  client is not told the open happened; the hook puts the daemon that owns the
  beamline in charge of preparing each engine. An Err from the hook fails
  the open and leaves the environment closed.