Add status query for total bytes used by a LevelDB instance

This commit adds a new LevelDB property "leveldb.total-bytes" that
returns the total bytes used by LevelDB for SSTs. This value is
equivalent to the sum of the reported per-level sizes shown in the
"leveldb.status" output.
change total from float to uint64_t
Merge pull request #74 from basho/jdb-total-bytes-stat

Add status query for total bytes used by a LevelDB instance