Fixed revision cache never releasing a store once cached.

This was causing the stores cached in the revision cache to grow continuously 
during the test suite. At the end, around 900 FMDatabase objects would still 
be present in-memory on Mac OS X, and even more on GNUstep.

Note: Accumulating unreleased database connections means we can hit the system 
opened file descriptor limit at some point, and get weird and random 
database opening errors.


git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10116 7b3f36aa-e6db-49c1-8b4c-66eac1997e64