insights: Track Forwards using HtlcEventStream

The newly added `HtlcEventStream` added to lnd's `routerrpc` provides new insight into a node's forwarding activity. This data is useful for a variety of applications - rate limiting for forwards, success rate tracking of various channels, rebalancing hints which trigger when we see a drop in incoming traffic or a rise in `ErrInsufficientBandwidth` failures.

This issue proposes the addition of a rounting package that consumes forwarding events, and matches them to their settle/fails where applicable. Such a solution could also write htlcs to disk, but for now we will start in memory and let requirements take form. A rough version of this change has been started [here](https://github.com/carlaKC/faraday/tree/temp-htlcnotifier).

### Steps to Completion
1. Update `lndclient` to consume forwarding events (in loop repo)
2. Consume htlc events, classifying them into forwards, receives and link failures (where forwards contain a forwarding event and subsequent settle/fail events.
3. Add the ability to make requests to this subsystem for a set of events over a period of time, which are handled by the main event loop consuming htlc events from lnd so we don't need any locking.

Start by reviewing the proposed temp-htlcnotifier branch and the lndclient integration around HtlcEventStream. Trace how the main event loop can classify forwarding events into forwards, receives, and link failures, then support period-based event queries without locking; completion follows the three listed steps.

Written by the indexing model from the issue text.