Combine tx and utxo requesters.
Combine tx and utxo requesters.

btcd now has only one notifier mechanism for received transactions to
a watched address.  This previously combines the seperate notifiers
for generic tx and exclusively unspent outputs.

A new handler will be added to watch for spent outputs instead of
relying on the now-removed "btcd:sendtx" notification.

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