Use READWRITEAS for CBlockHeader.
Use READWRITEAS for CBlockHeader.

Instead of casting pointers to read the CPureBlockHeader part of a
CBlockHeader, use the new READWRITEAS convenience macro.

namecoin/namecoin-core,namecoin/namecore,namecoin/namecoin-core,domob1812/namecore,namecoin/namecoin-core,pataquets/namecoin-core,pataquets/namecoin-core,pataquets/namecoin-core,domob1812/namecore,namecoin/namecoin-core,namecoin/namecore,pataquets/namecoin-core,namecoin/namecore,namecoin/namecoin-core,namecoin/namecore,namecoin/namecoin-core,domob1812/namecore,domob1812/namecore,namecoin/namecore,domob1812/namecore,domob1812/namecore,namecoin/namecore,pataquets/namecoin-core,pataquets/namecoin-core