Implement IndexProvider(for Mixed IndexType) class

Currently, there is no global index layer in s2graph. One idea to implement global index layer is using search engine and index Edge/Vertex based on their property with Index Configuration.

In this task, IndexProvider trait, which can be extended by Lucene, ElasticSearch, Solr, etc should be implemented and provide following methods on Edge/Vertex.

• Indexing

- def mutateVertices(vertices: Seq[S2Vertex]): Seq[Boolean]
- def mutateEdges(edges: Seq[S2Edge]): Seq[Boolean]

• Querying

- def fetchEdgeIds(queryString: String): java.util.List[EdgeId]
- def fetchVertexIds(queryString: String): java.util.List[VertexId]

I can't decide if return type of Querying APIs should be changed into Future or not so please give any help on this.