Allow setting fee limits on offerout

[jsarenik]

Issue and Steps to Reproduce

Currently both pay and keysend commands allow for setting fee limits. But when the node makes an offerout for 1sat, it may pay multiples of it on fees. I think it should be possible to limit the fees on the offerout so that it is not paid if no low-fee channel exists.

$ lightningd --version
v0.10.2-537-g560b090

getinfo output

{
   "id": "032de5c0f28f9d7d10c0c0b5ec92e83f9bf40def2bf40181c0f4330c57e58a8605",
   "alias": "IRATEMONKEY-v0.10.2-537-g560b090",
   "color": "60b090",
   "num_peers": 8,
   "num_pending_channels": 0,
   "num_active_channels": 8,
   "num_inactive_channels": 0,
   "address": [
      {
         "type": "ipv4",
         "address": "78.98.67.187",
         "port": 9735
      },
      {
         "type": "ipv6",
         "address": "300:5ecb:6b8a:d837::3",
         "port": 9735
      },
      {
         "type": "ipv6",
         "address": "2001:470:1f1b:365:aa20:66ff:fe3f:1909",
         "port": 9735
      },
      {
         "type": "torv3",
         "address": "seagd6ubksxmppexhzfibzjjk5ummiii5ancqdoc6ppfzhkqnrn4a5ad.onion",
         "port": 9735
      },
      {
         "type": "websocket",
         "port": 8524
      }
   ],
   "binding": [
      {
         "type": "ipv4",
         "address": "127.0.0.1",
         "port": 9735
      },
      {
         "type": "ipv4",
         "address": "192.168.1.118",
         "port": 9735
      },
      {
         "type": "ipv6",
         "address": "2001:470:1f1b:365:aa20:66ff:fe3f:1909",
         "port": 9735
      },
      {
         "type": "ipv6",
         "address": "300:5ecb:6b8a:d837::3",
         "port": 9735
      }
   ],
   "version": "v0.10.2-537-g560b090",
   "blockheight": 728794,
   "network": "bitcoin",
   "msatoshi_fees_collected": 0,
   "fees_collected_msat": "0msat",
   "lightning-dir": "/home/nsm/.lightning/bitcoin",
   "our_features": {
      "init": "80080a69a2",
      "node": "800080080a69a2",
      "channel": "",
      "invoice": "024100"
   }
}

[rustyrussell]

Hmm, it's not super-urgent, since the pay defaults still come into play AFAICT: we allow 0.5%, or 100% for payment less than 5 sats.