Remove a singleton value log buffer (#647)
Remove a singleton value log buffer (#647)

We don't need to keep this buffer around if no writes are going on. We just create a buffer as needed, and discard it afterward so Go GC can reclaim it, thus reducing memory usage.

* Remove the value log buf, using a buf local to write
* Use woffset instead of accessing writeableLogOffset directly.

dgraph-io/badger,dgraph-io/badger