search: support for static directory children

This change introduces a new index entry
to help with finding the children of a static directory.
It also fixes ResolvePrefixHop so that it takes
into account static directories, and not only collections.

This is the first step to support publishing static directories.

http://camlistore.org/issue/179

Change-Id: I5666e5caa6c782004054ae4c19a6b6119d4fda8b