Implement bulk key fetching and deleting for large queue without transaction

[ota42y]

Original PR #44

We want to reduce redis load by delete-queue command when the queue has a large number of tasks.
*a large number is about 10^6

I implemented following improvements:

• add --without-transaction option to large queue delete

  • if the user doesn't set this option, keep old feature.
  • if the queue have many tasks and without this option, raise error.

• use SScan instead of SMembers command to fetch task UIDs

  • 21b33e1

• use UnLink instead of Del command and bulk deleting keys by chunk size

  • https://redis.io/commands/UNLINK
  • 2b99cf7

• WIP points

  • Two pieces of code that are doing almost the same thing.
    • with / without transaction
  • Testing the corner case
    • i.e. abort and resume deleting
  • Performance test
    • I checked this PR work well for over 1234500 tasks
    • But it's take 20 minutes so it's not good for unite test.

fixes #28