Fixed some minors issues where the const qualifier was being discarded.
Merge pull request #218 from zhang-jingwang/2.4dev

Add plfs_flush and plfs_invalidate_cache to manage internal cache
added something about auto_cache
Merge pull request #223 from johnbent/2.4dev

added something about auto_cache
Merge pull request #219 from fintler/const-qualifier

Fixed some minors issues where the const qualifier was being discarded.
Merge branch 'master' into 2.4dev
Merge pull request #228 from agtorre/2.4dev

Merging updates from master to 2.4dev