Flatten DB

[scorbajio]

The aim of this change is to implement a flat database design for keeping state data in. This work will be integrated with the snap sync implementation for keeping snapshots of the latest 128 states. This work picks up where #780 left off. The goal is to first implement the StackTrie structure, then to implement the Snapshot and database wrapper necessary for data persistence.

[holgerd77]

Side note: VM is definitely the wrong package, that's clear I guess, assuming a bit this is just for testing? Just to point out, so that this doesn't "settle at the wrong place" 😋, I know though that this is very very early.

[codecov]

Codecov Report

Attention: Patch coverage is 60.51780% with 122 lines in your changes missing coverage. Please review.

Project coverage is 25.94%. Comparing base (4470cc3) to head (5e3c2d0).
Report is 35 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
block	66.20% <ø> (-7.25%)	⬇️
blockchain	84.92% <ø> (?)
client	0.00% <ø> (ø)
common	89.68% <ø> (?)
devp2p	0.00% <ø> (?)
evm	65.09% <ø> (?)
genesis	0.00% <ø> (?)
trie	54.65% <93.75%> (?)
tx	76.90% <ø> (-0.88%)	⬇️
util	66.40% <14.06%> (?)
vm	58.16% <80.00%> (?)
wallet	?

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