Replace tx IPC messages with candidates

[haydenshively]

Right now, each worker process sends mostly-complete transaction objects to the TxManager process via Node.js IPC. However, multiple worker processes can be matched up with a single TxManager/wallet. If this is the case, and both workers want to submit a liquidation transaction, the TxManager may as well combine the transactions into one. To do this, candidate objects should be sent over IPC instead of transactions.