Fixes #4299

Upon investigating this case further, we determined the issue to be a race between applying `JobBatchDeregisterRequest` fsm operation and processing job-deregister evals.

Processing job-deregister evals should wait until the FSM log message finishes applying, by using the snapshot index.  However, with `JobBatchDeregister`, any single individual job deregistering was applied accidentally incremented the snapshot index and resulted into processing job-deregister evals.  When a Nomad server receives an eval for a job in the batch that is yet to be deleted, we accidentally re-run it depending on the state of allocation.

This change ensures that we delete deregister all of the jobs and inserts all evals in a single transactions, thus blocking processing related evals until deregistering complete.