Refactor Oasis service for Graph and Place layer

[CodeBear801]

Subtask of #352(for all related tasks please go to https://github.com/Telenav/osrm-backend/issues/352#issuecomment-661453533), related with #363

Tasks:

• move selectionstrategy into solution
• create package of oasis/graph, move stationgraph, chargingstreategy, into graph. Create interface of

  generateOptimalChargeSolution

• create package of oasis/place, move connectivitymap, spatialindexer, stationconnquerier, stationfinder into it
• move internal/common into internal/entity
• move osrmconnector, searchconnector to integration/util
• change oasis to oasisd // move to next task