Issue 635: Stage4 to optionally generate persisted receipts (#644)

* Euphemerally -> Ephemerally

* Move StorageMode to ethdb and pass it to PrepareStagedSync

* linter

* Remove StorageModeThinHistory and move SetStorageModeIfNotExist into storage_mode.go

* Optionally write receipts in the execute stage

* memory profiler

* linter

* proper linter fix

* linter

* typo

* Merge stateDb with changeDb so that all-or-nothing is commited in stage_execute