Improved caching for DataStore

The current CachingDataStore implementation used with S3DataStore has certain problems:

• Lack of stats to show hit rate/miss rates for files being requested for downloads
• Lack of stats for async uploads
• CachingDataStore starts proactively downloading files in the background when a call to getRecord is made.
• Async upload functionality leaks into Backend implementations, LocalCache classes.
• The call to DataStore#getRecord() which makes multiple calls to backends which is problematic for S3 (i.e. when not being served bu cache)
• There is some functionality which is not used with Oak like length cache, sync/async touch etc. which can be removed and code simplified.