API to query status of async flushed datasets

[adammoody]

One application team has requested an ability to check when an async flush is complete and a way to know whether it succeeded or not. Really, there may be a similar need even for a sync flush.

One mechanism would be to provide an API allowing users to query the status of their datasets. In addition to flush status, there might be other information that one might be interested in:

• Provide a list of all known datasets. As one big list or one at a time? If providing one name at a time, like readdir, how to iterate through that list?
• Include flags assigned to dataset when it was created? SCR_FLAG_CHECKPOINT | SCR_FLAG_OUTPUT
• Is dataset in cache or on parallel file system?
• If in cache, should we provide the actual cache directory SCR_CACHE_BASE?
• SCR internals like dataset id, timestamp, byte counts, etc.

This is related to #390