routing: add additional indexes to the Route struct to allow for querying
routing: add additional indexes to the Route struct to allow for querying

This commit adds three new indexes to the Route struct. These indexes
allow a caller to check if a channel is in the route, check if a node
is in the route, query the next node after a target node, and query the
next channel after a target node. The combination of these new indexes
will allow the ChannelRouter to prune away routes from the available
set in response to any received errors.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd