* Allow sync reorg up to pruning history size
* Peer difficulty tracking
* Abort downloading block if received with NewBlock
* Set pruning history to 1200
* Renamed history size field
* move block queue to own module, a couple readability changes
* make block queue generic over verifiable data
also fixes heap size calculation
* make block queue into a more generic verification queue
* some module reoganization
* implement header queue
* clean up verification error messages