Daqd diagnostics

Closes #177 (closed).

Adds a signal to the daqd producer thread to signal if it has stalled. This is implemented in the shmem receiver which will signal when it has waited 2s for data.

Note the signal is '{PREFIX}_PRDCR_NOT_STALLED' instead of {PREFIX}_PRDCR_STALLED as we want this to default to stalled and only clear when data is flowing.

mentioned in issue #177 (closed)

mentioned in commit f9dcaf22

merged