Function send_start_instructions 

async fn send_start_instructions(
    workers: &Arc<Mutex<Vec<WorkerSender<Result<Instruction, Status>>>>>,
    measurement: &Arc<RwLock<Option<MeasurementState>>>,
    m_def: &ScheduleMeasurement,
    m_id: u32,
)

Builds Start instruction for all participating workers. Sends them for measurement init, and persists them for re-joining workers.