* Provide index to reconstruct missing storage slots
why;
Pivots will be changed anymore once they are officially archived. The
account of the archived pivots are ready to be swapped into the active
pivot. This leaves open how to treat storage slots not fetched yet.
Solution: when mothballing, an `account->storage-root` index is
compiled that can be used when swapping in accounts.
* Implement swap-in from earlier pivots
details;
When most accounts are covered by the current and previous pivot
sessions, swapping inthe accounts and storage slots (i.e. registering
account ranges done) from earlier pivots takes place if there is a
common sub-trie.
* Throttle pivot change when healing state has bean reached
why:
There is a hope to complete the current pivot, so pivot update can be
throttled. This is achieved by setting another minimum block number
distance for the pivot headers. This feature is still experimental
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