Extend GraphIndexer interface to support pagination

the findResources methods should support pagination by extending the interface with (from, to) arguments. From is inclusive, to is exclusive.

Background: Currently the idea of the maxhits parameter in GraphIndexer has been misunderstood. It can be used to support pagination in this manner:
First get results from 0-20 with maxhits 20. Then the user wants to see results from 20-40, so the same IndexSearcher can be used with maxhits set to 40. Because the IndexSearcher already holds results 0-19 in memory, only results 20-39 are loaded from the index.