CRM-20958 - Case, Activity BAOs - Watch out for saving stale timestamps

CRM-20958 - Case, Activity BAOs - Watch out for saving stale timestamps

There appears to be some application logic which follows a process like this:

 1. Read the case
 2. Tweak the data
 3. Save updated case

The problem is comes if step #4 resaves a timestamp loaded in step #1, which
is fairly likely to happen if you read+save the same record.

This was specifically observed on the "Manage Case" screen when editing
activities, but the data-flow is pretty common, so make a general fix to the
BAO.