EZP-21412: Improved insert/update of cache

See http://jira.ez.no/browse/EZP-21412.

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)

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