NBS is stable enough that we've made it the default store for command
line tools, and the go-to store for tests that require temporary, but
persistent, storage.
We intend to remove support for LevelDB-backed chunk storage
completely ASAP. This patch removes all usage of LevelDBStore from
noms.git, but doesn't remove LevelDBStore _just_ yet as there are
still some dependencies on it elsewhere.
Toward #3127