os: KeyValueDB: expose interface to obtain estimated store size

os: KeyValueDB: expose interface to obtain estimated store size

On LevelDBStore, instead of using leveldb's GetApproximateSizes() function,
we will instead assess what's the store's raw size from the contents of
the store dir (this means .sst's, .log's, etc).  The reason behind this
approach is that GetApproximateSizes() would expect us to provide a range
of keys for which to obtain an approximate size; on the other hand, what we
really want is to obtain the size of the store -- not the size of the
data (besides, with the compaction issues we've been seeing, we wonder
how reliable such approximation would be).

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>