[#503/506] Add Worker mutations and queries, update getRunStatus

[josephburgess]

Ticket [#503]
Ticket [#506]

Add methods to Repository class:

• createWorker - creates new worker
• getWorker - gets worker by workerId
• updateWorkerStatus - updates from PENDING > STARTED > COMPLETED and adds startedAt / completedAt timestamps
• getWorkersByRunId - gets all workers by runId
• getTestExecutionsbyRunId - get all testExecutions by runId
• getTestExecutionsByWorkerId - get all testExecutions by workerId

Update methods in Repository class

• updateTestRunStatus - adds completedAt timestamp when updating status to COMPLETED

Add Mutation resolvers:

• createWorkers(runId, executor, count) - creates {count} {executor} workers in database within a given {runId}
• setWorkerStatus - sets status of a given worker - does not allow status to be set to pending, or to be rolled back from started/completed
• refreshRunStatus - checks the status of all Workers within a run. If (allWorkersCompleted) - update runStatus to COMPLETED

Update Mutation resolver:

• getRun renamed to getRunStatus and added fields for total/pending/active/completed Workers

Add Query resolver:

• Query.worker for querying specific workers by ID

[dlawrynowicz]

looks good, few minor comments