Resolve "CI: timescan stops iterating"

Closes #3877 (closed)

Fix a race condition in the LimaAcquisitionMaster: the reading greenlet sets the ready event but does not go down yet when the next trigger is called. trigger sees that the reading greenlet is still running and does nothing. The reading greenlet exits soon after. The acquisition chain now waits for the ready event to be set but there is no reading loop running that can set it.