DurationSearcher shouldn't always convert duration strings as though they were time-tracking

To reproduce:

• Set up a Duration scripted field (non-time-tracking) such that it evaluates to 2 days for some issue (call it ISSUE-123).
• Ensure your time tracking configuration is set to 8 hours per day.
• Perform a JQL search for issues where the scripted field's value is
  > 4d.

Expected: ISSUE-123 is not included in the results.

Actual: ISSUE-123 is included.

This happens because the "4d" in the query gets indiscriminately converted in accordance with time tracking, and thus ends up representing a duration of 4*8 = 32 hours, which is less than 2 days.

We don't do the conversion ourselves, we delegate to JqlTimetrackingDurationSupport which as the name suggests is primarily for use with time-tracking queries.