Prevent "purge" to remove needed batch files in CompactibleFileStreamLog

I've seen couple of trials (in opened PRs, even I've also tried) which calls purge() in CompactibleFileStreamLog, but after looking at the codebase of CompactibleFileStreamLog, I've realized that purging latest compaction batch would break internal of CompactibleFileStreamLog and it throws IllegalStateException.

Given that CompactibleFileStreamLog maintains the batches and purges according to its configuration, it would be safer to just rely on CompactibleFileStreamLog to purge and prevent calling `purge` outside of CompactibleFileStreamLog.