make "solr.data.dir" points to multiple base path, and collection's replicas may distribute in them

As discussion in user mail-list，It's of worthy of making "solr.data.dir" points to multiple base paths, which collection's replicas may distribute in.

Currently， solr only could points to a single common dataDir，to maximize indexing throughput in same cases，solr maybe introduce a new feature to support multiple dataDir paths.