Implement JIT spec open channel flow

[JssDWt]

Identify forwards from the proposed JIT channel spec based on their SCID. If the SCID is persisted as a JIT channel request, fall into a channel flow that should be newly created as implementation of this issue.

• aggregate htlcs until all parts sum up to at least payment_size plus opening_fee
  • point of caution, think about restarts of lspd and/or the lightning node. Should be able to handle both replays of the same htlc when the node is restarted and missing replays of htlcs when lspd is restarted.
• Make sure htlc timeouts are handled appropriately, either by the node itself of by lspd explicitly. In either case, lspd will have to know about htlc expiry.
• keep track of when htlcs 'expire' through expiry of valid_until of the opening parameters.
• Make a decision on what to do with too many payment parts. Probably want to just take the excess amount?
• When there are enough payment parts, make sure the client is connected and valid_until has not expired.
• open the channel, when channel_ready is received, forward the htlcs with lowered amounts defined in the provided algorithm.
• Probably we won't do anything custom with the channel alias. We'll let the node specify a new channel alias for the channel open.
• Caution with htlcs that arrive after the decision to open the channel, those need to be handled as well.

[JssDWt]

Closing in favor of #109