[MONDRIAN-1998] Final fix for this case. We can (and should) safely change back the segment converter's pointers to the measures to hard references. When a RolapSchema gets cleaned by the GC (or it gets evicted from the pool), we cleanup all of the segments from the index. Once the straggling queries finish executing, the index entries are cleaned and the RolapSchema gets correctly picked by the GC. This was tested throughly via a profiler.