Goals

fix close functions post v2 upgrade

Implementation

Switch from checking status for transfer complete in transport as an indicator of a closed transfer to checking a direct function on channel state: TransferClosed()
Have existing transfer closing events set the transfer to be closed
Add an additional event, CloseTransfer, that dispatches to simply mark the transfer as closed without changing status
Have the closing functions only call CloseTransfer before actually updating the transport, and then call the final termination functions afterward.