EZP-21412: Improved insert/update of cache

### Jira issue
http://jira.ez.no/browse/EZP-21412.

### Description
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)

Should prevent misleading messages about race conditions, and perform better.

### Related PR
https://github.com/ezsystems/ezpublish-legacy/pull/724