Remove obsolete API functions
remove old API function.
Introduce an API change for btcdb

FetchTxBySha changes what it returns, it can now return a TxListReply and
and error if none are found.

FetchTxByShaList is renamed to FetchUnSpentTxByShaList to indicate that
it will (likey/eventually) only return Tx that have some unspent TxOuts.
Tx which are fully spent may not be (reliably) looked up using this API.