Renovate how we VSL log HTTP headers, so that all changes, including
unsets are logged.
The corresponding API changes

This makes the logging API work again after the changes to the Begin
and Link log records.

Even though restarts are linked below the request that initiated them,
the API will report them at the same level as the initiating
request. This is to help makes queries depending on levels
predictable.
Fix test cases after Begin and Link changes
Document the levels with regard to restarts
Fix test case