thresholds: fix issues with host based thresholds

thresholds: fix issues with host based thresholds

The flow manager thread (that also runs the host cleanup code) would
sometimes free a host before it's thresholds are timed out. This would
lead to misdetection or too many alerts.

This was mostly (only?) visible on slower systems. And was caused by a
mismatch between time concepts of the async flow manager thread and the
packet threads, resulting in the flow manager using a timestamp that
was before the threshold entry creation ts. This would lead to an
integer underflow in the timeout check, leading to a incorrect conclusion
that the threshold entry was timed out.

To address this,  check if the 'check' timestamp is not before the creation
timestamp.