Replace self._execution.signal with schedule_task
Replace self._execution.signal with schedule_task

This let us have ExternalWorkflowExecutionSignaled events in the
history, so we don't resend signals multiple times.

Signed-off-by: Yves Bastide <3b1fe340dba76bf37270abad774f327f50b5e1d8@botify.com>

botify-labs/simpleflow,botify-labs/simpleflow