tarantool / crud / 21022288690

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rebalance: move rebalance start from on_commit trigger to on_replace trigger

In order to prevent data duplication in MVCC usage case rebalance start was moved
from on_commit trigger to on_replace trigger.

This prevents situation like this:

1. vshard starts rebalancing and makes replace in `_bucket` space
2. this _bucket replace yields on commit
3. crud makes write request on storage, for example: insert in fast mode
4. this insert yields on commit
5. rebalancer starts pairing from `_bucket` space, does not see `insert`
   because it not commited yet. And processes bucket.
6. `_bucket` replace commits, starts safe mode
7. insert ends in fast mode
8. rebalancer moves bucket, which were inserted in fast mode
9. might happen duplicationrts rebalancing and makes replace in `_bucket` space

This happens because mode switches in `on_commit` trigger.

If mode switch happens in `on_replace` trigger:

1. vshard starts rebalancing and makes replace in `_bucket` space
2. in on replace trigger happens mode switch to safe mode
3. this _bucket replace yields on commit
4. crud makes write request on storage, for example: insert in safe mode
4. this insert yields on commit
5. rebalancer starts pairing from `_bucket` space and sees the ref
6. `_bucket` replace commits, starts safe mode
7. insert ends in safe mode
8. rebalancer handles refs properly

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