statfs cache

[carns]

This a feature suggestion rather than a bug:

We could cache statfs results in the client library. There only needs to be one per file system. The library could use the cached results any time its age is less than N seconds and the free block count is above a particular watermark.

This would be helpful for MPI_File_open() which triggers a statfs in the ad_bgl device. Upcoming versions will likely optimize this by only doing the statfs on one process, but a cache would still be helpful if man creates are done consecutively (as in a metadata benchmark).