Add checks and statistics around the truncate API

Summary
We would like to make sure the records are no longer present after truncated has been executed. This could be done in debug mode only through WT_ASSERT.

It would also be great to add statistics to know how records were truncated.

This will help better diagnose issues related to the truncate API.

Motivation

• Does this affect any team outside of WT?
  No
• How likely is it that this use case or problem will occur?
  Every time truncate is executed
• Is this issue urgent?
  Yes, we have issues that are hard to diagnose that involve the truncate API.

Acceptance Criteria (Definition of Done)

• Testing
  The existing tests might be enough. Just check the stats are generated and the WT_ASSERT are not getting fired.
• Documentation update
  No