* Relocated `IntervalSets` to nim-stew repo
* Accumulate accounts on temporary kv-DB
why:
Explore the data as returned from snap/1. Will be converted to a
`eth/db` next.
details:
Verify and accumulate per/state-root accounts downloaded via snap.
also:
Some unit tests
* Replace `Table` by `TrieDatabaseRef` for accounts accumulator
* update ticker statistics
details:
mean/variance based counter update
* allow persistent db for proved accounts
* rebase, and globally activate unit test
* fix statistics
Jordan Hrycaj