Implement failover modules. (failover queue/worker)

Queues are polled with an atomic 'lpopFailover' method to store a job on the
failover data-set(failoverQueue, failoverData). If a worker is terminated immediately while processing a job,
that job will be requeued eventually by FailoverWorker. Otherwise it will be just removed from the failover data-set.

Wheather worker terminated nomally or not(SIGTERM, SIGKILL, machine-failure, etc), failback will be run automatically.

And, Lua-script is used for transaction and reduction of latency.