client: MetadataCache: Added optimizations which allow to answer requests locally:

- If a path does not exist in the cached parent's directory listing, return that the path does not exist. Therefore the result value of GetStat() had to be changed.
- Added a function SimulateSetAttr() which allows to find out based on the cached stat entry if the SetAttr() call can be skipped.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3574 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534