set fee reserve to 0 if destination is us

[kiwiidb]

Closes #233
In case of an internal payment, the fee reserve should be 0.
This change allow clients to create a "Drain account" UX by combining different calls:

• make a payment (or more than 1) leaving just the fee reserve in the account, using a Lightning Address or 0 amount invoice.
• send the rest of the funds to some internal wallet (eg. hello@getalby.com or someone else).

[bumi]

can we add a test for this?

[kiwiidb]

Yes, done.