performance optimizations

```
Look through the code that accesses low-level indexes. Ensure that items are 
only kept in memory when they are needed to enhance performance, and objects 
are not re-created unnecessarily.

Classes include: 
SKOSSearcherImpl
IndexAdministrator
SKOSSchemeImpl
```

Original issue reported on code.google.com by `rscherle` on 9 Mar 2011 at 5:14
