Add support for reading connection(s) from linkahead-client.json

Summary

Clients based on the cpp lib are using a JSON config file, called linkahead-config.json (schema in cpplib). In the long run, all clients including the Python client should support the JSON config.

• Decide how to handle presence of both pylinkahead.ini and linkahead-config.json
• Decide how to handle multiple connections in linkahead-config.json
• Add support for reading the JSON config including validation using the schema
• Add tests for reading JSON config files
• Follow-up issues for other config options (logging, extension), and for extensions of client.json for advanced tools, crawler, pyinttest?

Specifications

• Version: 0.15
• Platform: any