Revisit DDL operations that are currently modifying config.chunks

We are only interested on DDL operations that are currently modifying the epoch/timestamp from config.chunks. I believe that there are only two operations: refineShardKey and resharding. We should check that we didn't break anything and whether we can remove some code from refineShardKey and perhaps resharding.

If we need to do any change on resharding we should properly synchronize with the NY Sharding team.