Merge pull request #49 from yifan-gu/execution

feat and fix(replica.go)
remove bool return in scanConflicts
Merge pull request #51 from fengjingchao/inst

remove bool return in scanConflicts
finish batch proposing
Merge pull request #52 from fengjingchao/inst

finish batch proposing
feat and refactor(replica.go)

1, Change the type of sccResult from list to slice.
2, Sort the instances in the scc in the order according
   to their rowIds and ids.
3, Batch the commands of the instances in scc together.
4, TODO: We still need to set the instance to be `executed` ATOMICALLY.