Add status query for total bytes used by a LevelDB instance

This pull-request 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.

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The main purpose of this change is for use in Riak through the `eleveldb:status` API to allow Riak to estimate the total bytes by a LevelDB vnode. The motivation being the new transfer progress work by @jrwest: basho/platform_tasks#23