Allow for multiple inputs/outputs in prepare_transaction and fulfill_transaction

[TimDaub]

Currently it is the following:

In [14]: cid = 0

In [15]: condition = creation_tx['transaction']['conditions'][cid]

In [16]: transfer_input = {
   ....:     'fulfillment': condition['condition']['details'],
   ....:     'input': {
   ....:          'cid': cid,
   ....:          'txid': creation_tx['id'],
   ....:      },
   ....:      'owners_before': condition['owners_after'],
   ....: }
   ....: 

In [17]: prepared_transfer_tx = bdb.transactions.prepare(
   ....:     operation='TRANSFER',
   ....:     asset=creation_tx['transaction']['asset'],
   ....:     inputs=transfer_input,
   ....:     owners_after=bob.verifying_key,
   ....: )
   ....: 

while it could be simply:
prepared_transfer_tx = bdb.transactions.prepare_transfer(input_tx, output_id, owners_after, metadata)
Notice that the operation, transfer_input, asset are all derived from the input_tx (full transaction, not only the tx_id). The complex transfer_input object is legacy of the common code, but can easily be abstracted away in the driver until the common code is fixed

[sbellem]

Can't we have transactions for which the inputs are composed of outputs from different "input transactions"?

[TimDaub]

Might be interesting to see how @iredjee is solving it in the js driver:
bigchaindb/js-bigchaindb-driver#98