Support c-lightning as swap client

[BitcoinOG]

Transferred from ExchangeUnion/xud#1281

Code base: https://github.com/ElementsProject/lightning
Docs: https://lightning.readthedocs.io/

Todo:

1. implement ClightningClient as SwapClient
2. there is no suitable hold invoice plugin to directly support our use case, but hold invoices are implemented by e.g. https://github.com/lightningd/plugins/blob/ea8e24eb5643db5c2693f4d24cb304a009533492/jitrebalance/jitrebalance.py & https://github.com/lightningd/plugins/blob/3290e4e45695393430dd9efe9ee767fa9f38e004/noise/noise.py (thanks @cdecker). Hold invoice hook is implemented since invoice hook ElementsProject/lightning#2540.
3. make an experimental btc/btc swap work with lnd or l-btc/btc swap using both on both ends

Notes:
Two ways to enforce the payment locktimes in c-lightning:

1. getroute + sendpay, basically the sendtoroute of lnd which we used before
2. encode the last hop cltv into the invoice which is generated by our hold invoice plugin

There doesn't seem to be a good way to set a cltv with the regular pay call (https://lightning.readthedocs.io/lightning-pay.7.html) according to docs & @cdecker

Ignore for now: L-BTC/Liquid support, backups etc

• Here the PR implementing Liquid support (only L-BTC, no Liquid Assets yet): elements: Add support of Liquid-BTC on elements ElementsProject/lightning#3078
• Slight differences in transaction and block formats will need adjustements in our locktime, delta, slippage calculations
• An updated fee estimation mechanism to account for the proofs and the additional fee outputs
  https://medium.com/blockstream/getting-started-with-lightning-on-the-liquid-network-8b5682752ffe (?)
• https://mm.fulmo.org/fulmo/channels/x-backup-plugin-c-lightning