Introduce staged sync (#482)

* Introducing staged sync

* Fix linter

* Fix linter

* Sync progress in the DB

* Fix linter

* Fix linter

* Not to wake body and receipt downloaders in the header stage

* Next stage

* Fix tests

* Fix linter

* More on block bodies downloading

* More on downloading bodes

* Remove debug

* Fix test compilation

* Fix tests

* Fix linter

* Fix linter

* Turn off the miner during staged sync

* More fixes