Implement non-strict forwarding

Implement non-strict forwarding

This change implements non-strict forwarding, allowing the node to
forward an HTLC along a channel other than the one specified
by short_channel_id in the onion message, so long as the receiver has
the same node public key intended by short_channel_id
([BOLT](https://github.com/lightning/bolts/blob/57ce4b1e05c996fa649f00dc13521f6d496a288f/04-onion-routing.md#non-strict-forwarding)).
This can improve payment reliability when there are multiple channels
with the same peer e.g. when outbound liquidity is replenished by
opening a new channel.

The implemented forwarding strategy now chooses the channel with the
lowest outbound liquidity that can forward an HTLC to maximize the
probability of being able to successfully forward a subsequent HTLC.

Fixes #1278.