* Piecemeal trie inspection
details:
Trie inspection will stop after maximum number of nodes visited.
The inspection can be resumed using the returned state from the
last session.
why:
This feature allows for task switch between `piecemeal` sessions.
* Extract pivot helper code from `worker.nim` => `pivot_helper.nim`
* Accounts import will now return dangling paths from `proof` nodes
why:
With proper bookkeeping, this can be used to start healing without
analysing the the probably full trie.
* Update `unprocessed` account range handling
why:
More generally, the API of a pairs of unprocessed intervals favours
the first set and not before that is exhausted the second set comes
into play.
This was unfortunately implemented which caused the ranges to be
unnecessarily fractioned. Now the number of range interval typically
remains in the lower single digit numbers.
* Save sync state after end of downloading some accounts
details:
restore/resume to be implemented later
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