add a "modified_seconds" index.

The precision of the standard "modified" catalog index is minutes.
For sensitive features we need a more precise index.

Running queries against the "modified_seconds" is more expensive
than running them against the default "modified" index - but
also more precisie.

We need the precise index as repository tree cache key.
use the modified_seconds index as cache key for the navigation.
Merge pull request #565 from 4teamwork/jone-tree-caching-fixes

Tree Portlet: improve cache key precision