When removing many Graphs and Data Sources, polling cycles may overrun

[TheWitness]

Describe the bug

Experienced this today. Basically, the items can not be removed from the poller_output table if the local_data entries have been removed. This results in the poller process timing out.

To Reproduce

Steps to reproduce the behavior:

1. Bulk remove thousands of graphs

2. Watch the poller processes end

3. After all poller processes have ended, you still see entries in the poller_output table

4. See error

Expected behavior

Cacti should periodically prune these entries instead of timing out.

[TheWitness]

@bmfmancini this might have something to do with missing notifications. Not sure, but it could.