Refactor write-write conflict resolving logic

Current implementation for resolving write-write conflict on snapshotEdge is overwhelmingly complicated, so I am suggesting refactor this to make it easy to understand.

Current master test cases failed with positive fail probability for RPC(hbase.fail.prob=0.01).

I can reproduce it by running `sbt "project s2core" -Dconfig.file=s2rest_play/conf/test.conf test`.

We can think this problem as critical section problem. Multiple thread that want to mutate same SnapshotEdge can only proceed one-by-one to keep consistent view on related IndexEdge. To achieve this, we are storing extra data on SnapshotEdge.

Details are on gitbook.

I think following 3 things would be expected output after resolving this issue.

1. more comments and simplified code.
2. remove excessive re-fetch on every retry from partial failure. since it is guaranteed that only one thread can proceed at a time, re-fetch is not necessary.
3. remove Thread.sleep between retry from partial failure.