Sweep updates timestamp of last extracted scan rather than the time of the last sweep

[nleach999]

Describe the problem

When CxAnalytix performs a sweep, it will calculate all scans that need to be extracted based on a recorded timestamp. The timestamp of the current sweep is then recorded for use as the starting time for the next sweep.

If the current sweep is interrupted or a scan can't be extracted, scans that were resolved in the current sweep may never be extracted.

Proposed solution

Update the last sweep timestamp per project using the timestamp of the last scan extracted for that project. This should mean that scans for each project are extracted in chronological order.

[nleach999]

Implementation of this depends on #52.