fix: used to sync log to leader's commit which lags behind applied

This used to cause flakyness of test_bucket_rebalancing_respects_replication_factor
like this:

- governor proposes entry at index N with weight updates,
  and waits till it's applied

- commit index is not updated yet, but governor calls rpc::sharding to
  update the weights in the vshard subsystem on instances and passes the
  lagging commit index for syncing

- peers sync up to the lagging commit index and then reconfigure vshard
  to the outdated weights

- governor thinks everything is in order and waits for updates

- the test fails because buckets don't get rebalanced to the storages
  with weights set to 0