Parametric route search

[araspitzu]

This PR aims at enabling a narrower search for routes by exposing a set of parameters to limit the result and the search scope. Routes can now be 'requested' imposing a maximum number of hops, a maximum total CLTV and a maximum fees, this information will be actively used to prune edges when searching the graph for a route.

[araspitzu]

Last addition to this PR: config key eclair.max-payment-fee has been removed in favor of router.search-max-fee-pct 61e7174, the two had near identical purpose and were overlapping with the latest change. CC: @dpad85

[pm47]

I propose the following change for the configuration parameters (names, description and units). What do you think @araspitzu?

// parameters for the path finding algorithm when looking for a payment route
path-finding {
  max-route-length = 6 // max route length for the 'first pass', if none is found then a second pass is made with no limit
  max-cltv =  1008 // max acceptable cltv expiry for the payment (1008 ~ 1 week)
  fee-threshold-sat = 21 // if fee is below this value we skip the max-fee-pct check
  max-fee-pct = 0.03 // route will be discarded if fee is above this value (in percentage relative to the total payment amount); doesn't apply if fee < fee-threshold-sat 
}

[araspitzu]

I propose the following change for the configuration parameters (names, description and units). What do you think @araspitzu?

// parameters for the path finding algorithm when looking for a payment route
path-finding {
  max-route-length = 6 // max route length for the 'first pass', if none is found then a second pass is made with no limit
  max-cltv =  1008 // max acceptable cltv expiry for the payment (1008 ~ 1 week)
  fee-threshold-sat = 21 // if fee is below this value we skip the max-fee-pct check
  max-fee-pct = 0.03 // route will be discarded if fee is above this value (in percentage relative to the total payment amount); doesn't apply if fee < fee-threshold-sat 
}

I like them, path-finding is a good descriptive name. Mostly we want to make sure there is no confusion with other config keys (i.e max-cltv) and i think path-finding does a good job distinguishing them. Props for the fee threshold in satoshi too, it's a bit better now.

[araspitzu]

Addressed comments in c9f93e2