rebalance overhaul: internal pathfinding, economic counterparties

[bitromortac]

This is quite a large redesign of the rebalance functionality (related #98). The main changes are:

• we now rely on internal pathfinding: this gives more freedom in testing other pathfinding heuristics
  • we add hints to know which channels have enough capacity, by tracking successful and failed parts of routes
  • failed nodes are penalized by closeness to the failing hop
  • the time to forward an HTLC is measured and taken into account for route construction
• rebalancing is restructured, the largest change is that we now select the route with the lowest weight (liquidity hints + fees), among all counterparty channels instead of rebalancing counterparty channels one-by-one
• the rebalance amount is gradually reduced (split into half) and upon payment success we continue with the amount left to increase rebalance success
• counterparty rebalance channels are selected in such a way that a positive future gain is expected (disregarding rebalance costs -- it's possible that it's slightly negative) (can be turned off with the --force flag)
• the circle command is removed (there is lncli functionality for this)
• tests are added for liquidity hints and the new rebalance command

Steps for completion:

• persist liquidity hints
• all counterparty channels should get a chance to be tried, by gradually removing already tried counterparty channels from the process
• reasonable initial pathfinding heuristics
• reflect channel reserve and ctx fees correctly
• automatic direction setting and amount sanity check
• track elapsed time until fail/success
• pubkey-based channel selection
• response-time penalty
• think about negative gain
• update readme
• forget hints after some time