ETCM-463: Add PeerStatisticsActor to track message counts

[aakoshh]

Description

Allow #833 to prioritise which peers to prune by counting the number of messages coming from them in a fixed time window.

Proposed Solution

Adds a TimeSlotStats counter to the PeerManagerActor, which now subscribes to MessageFromPeer events by the event bus and adds 1 to the peer every time we receive a response for a request we sent. Doesn't increment the counter for requests sent by the other side, although that could make sense too, depends on what we consider a valuable connection: one that serves us data, or one that makes its presence known by requesting it.

Some of this can be abused though, like NewBlockHashes, if it sends the same notification repeatedly, or the notifications about non-existing hashes; these conditions should be monitored on a different level and penalised. I'm guessing that would mean another component would have to publish events to the message bus about such faults, which would be a new event type.

The TimeSlotStats can be used with any statistic that can be merged, so we can always extend it to include faults, data size; less obvious would be the response time, at least not with this subscription, but perhaps the PeerActor can send messages to its parent directly instead.

The idea is that we can call peerMessageStats.getAll() when we want to prune and use it to provide an ordering for which peers to prune first: the ones which we haven't pulled any data from in the last 30 minutes for example.

(I added the counter as a var to the actor, seemed like there would be quite some noise if I introduce some kind of state wrapper for all the places context become ... occurs, but let me know if that's preferred).

[KonradStaniec]

@lemastero @biandratti @enriquerodbe as you will taking over project soon, I have added you as a reviewers so you will be aware what is happening here