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