Progress so far in setup for backend in memory test queues

The following pull request is the progress Krishnan and I have made so far with the in backend memory test queues.

What we have so far:

Calling the CassandraBackend constructor will 
1. Retrieve all test names (prefix and title) from Cassandra
2. Retrieve latest test scores corresponding to retrieved test prefixes and titles
3. Order retrieved tests by placing them in a priority queue

Testing done so far:
Inserted 1 commit (commits table in schema), 2 tests (tests table in schema) and 2 tests with score (test_by_score in schema) in local Cassandra store. Ordering seems fine on the test scores in the priority queue. 

Next steps:

Find some way to insert dummy data into local Cassandra store ( Perhaps looking at how initAll.sh does it might be helpful? ) and then verify that this change behaves as expected. Once we have dummy data working progress can then be made in frontend - backend integration, test retry logic etc.