Pass Route to send_payment as a reference, not move

Pass Route to send_payment as a reference, not move

ChannelManager::send_payment stopped utilizing its ownership of the
Route with MPP (which, for readability, now clone()s the individual
paths when creating HTLCSource::OutboundRoute objects). While this
isn't ideal, it likely also makes sense to ensure that the user has
access to the Route after sending to correlate individual path
failures with the paths in the route or, in the future, retry
individual paths.

Thus, the easiest solution is to just take the Route by reference,
allowing the user to retain ownership.