Change the way the nav-cache is rebuilt.

Change the way the nav-cache is rebuilt.

Instead of deleting all table contents, actually remove the entire file on disk and re-create. This is fractionally more work, but removes any possibility of stale indices or missing deletes causing clutter after rebuilds. My suspicion is, this is cause the erratic performance some people have seen with the airports search dialog, so will back-port to 2.10.