[Hivemind] Add fetch_obj_status method to GithubObjectExtraction class

[cyri113]

Part of the Github Vectorize (Summary). Please read this document before starting.

Given an array of pr and issue ids, using the pre-built neo4j connection, return the status of the prs and issues as a dictionary id -> status.

Tasks

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1. Write a public method fetch_obj_status in GithubObjectExtraction
2. Write test cases

The class should have 100% coverage on code climate.

#102 should be completed first.