Fixed dogecoin reorg bug that caused invalid work error due to BlockTree::height() returning negative values for non trunk blocks.

Added serialization test stub
added protocol_version method to ponzicoin example
Fixed Namecoin reorg bug - when reorganizing and a name reservation is part of the reorganization block, the entry in the names table would not get deleted properly:
* DELETE FROM Names WHERE coin = (SELECT coin FROM Unspents WHERE ocnf = ?)
will only work if there is exactly one coin in the confirmation in question.
Changed it to:
* DELETE FROM Names WHERE coin IN (SELECT coin FROM Unspents WHERE ocnf = ?)
Which will delete the any of occurrence (notice: ‘=‘ -> ‘IN’)
Merge branch 'master' of https://github.com/libcoin/libcoin
Merge branch 'master' into name-show