MRC backend: re-arranged metaddata management, so that X-Loc lists and...

MRC backend: re-arranged metaddata management, so that X-Loc lists and rarely-changed metadata can be retrieved with a single database lookup

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@705 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534