Backport code structure changes

[seunlanlege]

Ok, so the master branch and stable/beta branches have diverged significantly in code structure so much so that backporting #11212 felt very surgical. Crates like client-traits, engines etc don't exist on the stable/beta. So any PR's that depend on that code would be a nightmare to backport.

So basically we need to backport these PR's so backporting future PR's based on master to stable

• Extract engines to own crates #10966
• Extract the Engine trait #10958
• Extract Machine from ethcore #10949
• Move more types out of ethcore #10880
• Move DatabaseExtras back to trace #10868
• Extract state-db from ethcore #10858
• Move more code into state-account #10840
• Extract AccountDB to account-db #10839
• Extricate PodAccount and state Account to own crates #10838
• Break circular dependency between Client and Engine (part 1) #10833

[niklasad1]

/cc @dvdplm is this worth the effort?

EDIT: I think this would require us to cherry-pick almost all commits but I might be wrong

[dvdplm]

No, definitely not worth it. The upcoming 2.5.10 is the last in the 2.5 series and we’re mainly concerned with the Istanbul block numbers. Anything else is “backport if easy, else skip it”.