Implement bulk key fetching and deleting for large queue

[shioshiota]

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:

• 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

fixes #28

[everpeace]

I left several comments. PTAL.

[everpeace]

Would you mind adding tests for chunked deletion (large queue)??

[shioshiota]

I added a test for chunked deletion.
f2b8503