routing/router: enable MPP sends for SendToRoute
routing/router: enable MPP sends for SendToRoute

This commit enables MPP sends for SendToRoute, by allowing launching
another payment attempt if the hash is already registered with the
ControlTower.

We also set the total payment amount of of the payment from mpp record,
to indicate that the shard value might be different from the total
payment value.

We only mark non-MPP payments as failed in the database after
encountering a failure, since we might want to try more shards for MPP.
For now this means that MPP sendToRoute payments will be failed
only after a restart has happened.

LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd