Allow sending of PFSCapacityUpdate along with PFS request

[karlb]

We currently expect both channel participants to regularly update the PFS with their respective capacities. This results in two limitations:

• Privacy conscious clients can't reliably use the PFS (this is why we have no such --routing-mode at the moment)
• Clients have to frequently sign PFSCapacityUpdate messages if they want to use a PFS, which is a problem for light clients that use interactive signing.

Both of these problems could be avoided if clients were allowed to include their current capacity in a PFS request, so that the PFS can update their capacity before calculating the route and fee estimate.

This would help with interactive signing because the capacity update is included in the PFS request, which is already being signed. So no additional signing would be necessary.

For privacy conscious clients this would provide them with a way to use PFSs while only sharing their capacity with a single PFS and doing so only at those times at which they want to initiate a payment.

Unsolved problem: the fee estimate for the target's channel can still be inaccurate when the target does not send PFS updates.

See also #736.