EZP-21412: Improved insert/update of cache

Implements _insertUpdate in the Oracle DFS handler in order
to better handle race conditions.
- only updates entries that weren't changed (mtime) since
  we checked for their existence (better performances)
- silently (jerken-clustering notice) ignores insert failures
  on duplicate key
- doesn't store nor end generation when no cache has been written
  to DFS (better performances)