Cursor alive checking

Otherwise slow query every 2 seconds
Merge branch 'master' of https://github.com/YuriyIlyin/mongo-connector into github_5

Conflicts:
	mongo_connector/oplog_manager.py
accidentally removed a try-except block during doc manager upsert
cleaned up while loop to iterate during cursor lifetime
forgotten : at start of while loop
Merge branch 'github_5'