candidate fix not complete

[dustinxie]

What version of iotex-core image (or code branch) are you using?

c316b81

What did you expect to see?

1. dry-run the candidate fix couple of times
2. it should generate same digest each time

What did you see instead?

it can generate diff digest, see https://www.diffchecker.com/gMQfaast line 483

Root-cause: c316b81#diff-4c37354767dc7764b93f5ed545905b6ee391187134ad8f29c09a7c71dfc51518R53-R60 added 2 duplicate candidates to retval, and sort.Sort(retval) can produce different order

Suggested fix:

1. just call candCenter.Upsert() for these 2 duplicate candidates, no need to sort the list, and no need to add them to retval
2. it is better to keep sort.Sort(cands) inside flush(), i.e., revert this: c316b81#diff-4c37354767dc7764b93f5ed545905b6ee391187134ad8f29c09a7c71dfc51518L124