Fix issue where when connecting and disconnecting a TX

When connecting an tx output to an input, the TransactionInput#getValue() always returns null.
Also the output.getSpentBy() returns null, while the expected behavior is to return the connected TransactionInput object.

Could not run all the tests, as it stucks with this: ProtobufParser.timeoutOccurred: Timeout occurred for org.bitcoinj.protocols.channels.PaymentChannelServerListener$ServerHandler