Failover feature should be worked in regardless of order in slaves.json

[walkingtospace]

For example,

A - master
B - slave

When A is down, B becomes a master. Then we execute A as a slave and A recognize B as a master. It works well.
But when B is down, A should be a master, but it isn't since the order of A is above of B at slaves.json.

Therefore, we need more flexible scheduling for the ordering.

[walkingtospace]

I think circular-queue would be a solution.