Download v2 fixes (#1903)

* [download.v2] Fix endianness of Difficulty in Status message

* Add comments

* Finish stage to use RwTx

* Print header age in stage 1

* Print new block messages

* TopSeenHeight to be updated from any header

* block in the future handling and skip the test

* Fix downloader test

* Fix test

Co-authored-by: Alexey Sharp <alexeysharp@Alexeys-iMac.local>
Co-authored-by: Alex Sharp <alexsharp@Alexs-MacBook-Pro.local>