Badger can take 1000x the amount of needed space if not GC'ed or compacted
(#1320), even for non heavy usage. Cluster has no provisions to run datastore
GC operations and while they could be added, they are not ensured to
help. Improvements on Badger v3 might help but would still need to GC
explicitally.
Cluster was however designed to support any go-datastore as backend.
This commit adds LevelDB support. LevelDB go-datastore wrapper is mature, does
not need GC and should work well for most cluster usecases, which are not
overly demanding.
A new `--datastore` flag has been added on init. The store backend is selected
based on the value in the configuration, similar to how raft/crdt is. The
default is set to leveldb. From now on it should be easier to add additional
backends, i.e. badgerv3.
