jsonrpc: list 0 confirmation txs for listsinceblock.
jsonrpc: list 0 confirmation txs for listsinceblock.

This updates listsinceblock to allow listing 0-conf txs and
corrects the last_block result field behaviour.
jrick/btcwallet,jrick/dcrwallet,jrick/dcrwallet,decred/dcrwallet,decred/dcrwallet,jrick/btcwallet