[wallet] Flatten CWalletTx class hierarchy
[wallet] Flatten CWalletTx class hierarchy

Removes CMerkleTx as a base class for CWalletTx. Serialization logic is
moved from CMerkleTx to CWalletTx.

DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin