+ *FSDB.HasKey now uses common.FileExists to test for file existence
+ IsKeyInDomain takes key as a []byte slice instead of as a string
to avoid extraneous []byte<-->string conversions for start and end
+ Iterator.Close() instead of Iterator.Release()
+ withDB helper to encapsulate DB creation, deferred cleanups
so that for loops can use opened DBs and discard them ASAP
Addressing accepted changes from review with @jaekwon
