Improve Evenscheduler to schedule executors based supervisor used slots

[fxjwind]

• fix wrong comment
• improve the logic of sort-slots #359

[patricioe]

Is this solving the problem of no allocating evenly based on currently used slots?

In my case, rebalance command keeps on sending the worker to the same machine instead of the one of all free slots.

[xumingming]

the test data here actually violates the worker number constraints, you specified the worker number to 4 for topology1, but here it is assigned 6 different workers.

[fxjwind]

@xumingming , thanks for your comments, have already updated

[fxjwind]

@patricioe , you are right, this fix will choose supervisor with less workers(used-slots) preferentially.