Still some more optimizations

1. added a check to prohibit purge at all, if the whole cache store does not contain any expireable cache entry (works great with non-expireable cache entries - like Lucene indexes)
2. added some more configuration options for purge:
2.a. minimum time to wait between to purge runs
2.b. minimum time to wait after last update to a bucket before purge run
2.c. maximum time to wait after the last purge run
3. prohibit purge run, if no update occured
4. prohibit purge test (and execution), if bucket file did not change after last purge run

Will attach source code.