Add sendpayment calls to SwapClient

[kilrau]

Since we can expect users wanting to send regular lightning/raiden payments and L2 payment calls are not easy to use/learn (e.g. on raiden) we want to expose them in a simplified manner via xud's grpc and CLI as well:

Add to SwapClient interface, implement for LNDClient only (for now):

• pay <Invoice>
  LND Call: https://api.lightning.community/#sendpaymentsync
• send <amount> <currency> <XudPubKey/LndPubKey/0xAddress>
  LND Call: EDIT: available as --keysend in https://api.lightning.community/#sendpayment (even though I couldn't make it work in some tests)

[kilrau]

Not that interesting for lnd, since lncli can be comfortably used from within xud ctl, but very interesting for raiden.

[engwarrior]

Pls provide estimate for this task.

[kilrau]

I'd want to postpone this for ~one month. @engwarrior