Statement Distribution Per Peer Rate Limit (#3444)

- [x] Drop requests from a PeerID that is already being served by us.
- [x] Don't sent requests to a PeerID if we already are requesting
something from them at that moment (prioritise other requests or wait).
- [x] Tests
- [ ] ~~Add a small rep update for unsolicited requests (same peer
request)~~ not included in original PR due to potential issues with
nodes slowly updating
- [x] Add a metric to track the amount of dropped requests due to peer
rate limiting
- [x] Add a metric to track how many time a node reaches the max
parallel requests limit in v2+

Helps with but does not close yet:
https://github.com/paritytech-secops/srlabs_findings/issues/303