CMerkleTx only exists as a base class for CWalletTx and for wallet file
serialization/deserialization. Move CMerkleTx methods into CWalletTx,
but leave class hierarchy and serialization logic in place.
https://github.com/bitcoin/bitcoin/pull/16451/commits/b3a9d179f23f654b8fba63924bbca5fd31ad4bb0
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Partial backport of Core PR16451