Enable passing custom go-datastore implementation into threads initializer

[carsonfarmer]

Some folks would like to explore using https://github.com/ipfs/go-ds-leveldb rather than then default badger datastore for various memory issues. Right now, buckets and other external libs that use go-threads rely on the DefaultNetwork initializer to create the threads network layer. This in turn relies on the default ipfslite datastore, which is badger. Since go-ds-leveldb also implements the datastore batching interface, it would be useful to be able to pass this all the way down to the ipfslite initializer. Perhaps we could expose this via some of our config options?