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* Relocated mothballing (i.e. swap-in preparation) logic details: Mothballing was previously tested & started after downloading account ranges in `range_fetch_accounts`. Whenever current download or healing stops because of a pivot change, swap-in preparation is needed (otherwise some storage slots may get lost when swap-in takes place.) Also, `execSnapSyncAction()` has been moved back to `pivot_helper`. * Reorganised source file directories details: Grouped pivot focused modules into `pivot` directory * Renamed `checkNodes`, `sickSubTries` as `nodes.check`, `nodes.missing` why: Both lists are typically used together as pair. Renaming `sickSubTries` reflects moving away from a healing centric view towards a swap-in attitude. * Multi times coverage recording details: Per pivot account ranges are accumulated into coverage range set. This set fill eventually contain a singe range of account hashes [0..2^256] which amounts to 100% capacity. A counter has been added that is incremented whenever max capacity is reached. The accumulated range is then reset to empty. The effect of this setting is that the coverage can be evenly duplicated. So 200% would not accumulate on a particular region. * Update range length comparisons (mod 2^256) why: A range interval can have sizes 1..2^256 as it cannot be empty by definition. The number of points in a range intervals set can have 0..2^256 points. As the scalar range is a residue class modulo 2^256, the residue class 0 means length 2^256 for a range interval, but can be 0 or 2^256 for the number of points in a range intervals set. * Generalised `hexaryEnvelopeDecompose()` details: Compile the complement of the union of some (processed) intervals and express this complement as a list of envelopes of sub-tries. This facility is directly applicable to swap-in book-keeping. * Re-factor `swapIn()` why: Good idea but baloney implementation. The main algorithm is based on the generalised version of `hexaryEnvelopeDecompose()` which has been derived from this implementation. * Refactor `healAccounts()` using `hexaryEnvelopeDecompose()` as main driver why: Previously, the hexary trie was searched recursively for dangling nodes which has a poor worst case performance already when the trie is reasonably populated. The function `hexaryEnvelopeDecompose()` is a magnitude faster because it does not peruse existing sub-tries in order to find missing nodes although result is not fully compatible with the previous function. So recursive search is used in a limited mode only when the decomposer will not deliver a useful result. * Logging & maintenance fixes details: Preparation for abandoning buddy-global healing variables `node`, `resumeCtx`, and `lockTriePerusal`. These variable are trie-perusal centric which will be run on the back burner in favour of `hexaryEnvelopeDecompose()` which is used for accounts healing already.