Update Connection Pool implementation

- Fixd issue when VirtuosoPreparedStatement.close() is called twice (PreparedStatement Finalizer could hang)
- Removed deprecated PrepredStatement cache from ConnectionWrapper
- ConnectionPool cache was updated for more optimal using of locks
- Fix issue https://github.com/openlink/virtuoso-opensource/issues/243
  (Deadlock in JDBC Virtuoso connection pool)
Merge branch 'develop/6' into develop/7