* Add quick hexary trie inspector, called `dismantle()`
why:
+ Full hexary trie perusal is slow if running down leaf nodes
+ For known range of leaf nodes, work out the UInt126-complement of
partial sub-trie paths (for existing nodes). The result should cover
no (or only a few) sub-tries with leaf nodes.
* Extract common healing methods => `sub_tries_helper.nim`
details:
Also apply quick hexary trie inspection tool `dismantle()`
Replace `inspectAccountsTrie()` wrapper by `hexaryInspectTrie()`
* Re-arrange task dispatching in main peer worker
* Refactor accounts and storage slots downloaders
* Rename `HexaryDbError` => `HexaryError`
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