Have listsinceblock return a btcjson result type.
Have listsinceblock return a btcjson result type.

ok @jrick

synrg-labs/btcwallet,jrick/btcwallet,Stash-Crypto/btcwallet,Stash-Crypto/btcwallet,btcsuite/btcwallet,jrick/dcrwallet,Roasbeef/btcwallet,synrg-labs/btcwallet,Roasbeef/btcwallet,decred/dcrwallet,btcsuite/btcwallet,jrick/dcrwallet,decred/dcrwallet,jrick/btcwallet