Remove taffydb file from FS

Hi,

continuing on my change to use taffydb for caching, I need the ability to get rid of the cache.  Here is a patch for it. Basically, removes file from file system and purge the data in-memory (so that the "cached" data is in sync between FS and memory).  I added an option to prevent the actual purge of data for completeness sake, even though I am not sure it is really helpful.

regards
-t