Oasis - Refactor station finder

[CodeBear801]

This task is part of #240

The idea of package stationfinder is finder near by charge stations for given location. It originally supports search by Telenav search service only, after we implement search based on google:s2, we want to provide a general interface for finder's users

[CodeBear801]

/*

Package stationfinder provide functionality to find nearby charge stations and
related algorithm.
It's find functionality could be achieved by:
- TNSearchFinder, which is implemented by Telenav's search web service.
  + The data source is from professional data
  + Based on Lucence and hilbert value
  + Could potentially supports dynamic information

- LocalIndexerFinder
  + Support any kind of data sources as long as following format defined in https://github.com/Telenav/osrm-backend/issues/237#issue-585201041
  + For Telenav usage, uses the same source with Telenav search service
  + Spatial index is build based on google s2


*/

[CodeBear801]

Some new item need to change:

• change the name of tnsearchfinder to cloudfinder
• change name of NewTnSearchStationFinder to New
• add protection code in go func() in unit test of orig_station_finder_test.go

More information could go to here: #271