* Split fetch accounts into sub-modules
details:
There will be separated modules for accounts snapshot, storage snapshot,
and healing for either.
* Allow to rebase pivot before negotiated header
why:
Peers seem to have not too many snapshots available. By setting back the
pivot block header slightly, the chances might be higher to find more
peers to serve this pivot. Experiment on mainnet showed that setting back
too much (tested with 1024), the chances to find matching snapshot peers
seem to decrease.
* Add accounts healing
* Update variable/field naming in `worker_desc` for readability
* Handle leaf nodes in accounts healing
why:
There is no need to fetch accounts when they had been added by the
healing process. On the flip side, these accounts must be checked for
storage data and the batch queue updated, accordingly.
* Reorganising accounts hash ranges batch queue
why:
The aim is to formally cover as many accounts as possible for different
pivot state root environments. Formerly, this was tried by starting the
accounts batch queue at a random value for each pivot (and wrapping
around.)
Now, each pivot environment starts with an interval set mutually
disjunct from any interval set retrieved with other pivot state roots.
also:
Stop fishing for more pivots in `worker` if 100% download is reached
* Reorganise/update accounts healing
why:
Error handling was wrong and the (math. complexity of) whole process
could be better managed.
details:
Much of the algorithm is now documented at the top of the file
`heal_accounts.nim`
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