Rate limit RPC requests (#1402)

## Issue Addressed
#1056 

## Proposed Changes
- Add a rate limiter to the RPC behaviour. This also means the rate limiting occurs just before the door to the application level, so the number of connections a peer opens does not affect this (this would happen in the future if put on the handler)
- The algorithm used is the leaky bucket as a meter / token bucket implemented the GCRA way
- Each protocol has its own limit. Due to the way the algorithm works, the "small" protocols have a hard limit, while bbrange and bbroot allow [burstiness](https://www.wikiwand.com/en/Burstiness). This is so that a peer can't request hundreds of individual requests expecting only one block in a short period of time, it also allows a peer to send two half size requests instead of one with max if they want to without getting limited, and.. it also allows a peer to request a batch of the maximum size and then send _appropriately spaced_ requests of really small sizes. From what I've seen in sync this is plausible when reaching the target slot.

## Additional Info
Needs to be heavily tested

beacon_node/eth2_libp2p/src/peer_manager/mod.rs

@@ -331,6 +331,7 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
                 RPCResponseErrorCode::Unknown => PeerAction::HighToleranceError,
                 RPCResponseErrorCode::ServerError => PeerAction::MidToleranceError,
                 RPCResponseErrorCode::InvalidRequest => PeerAction::LowToleranceError,
+                RPCResponseErrorCode::RateLimited => PeerAction::LowToleranceError,
             },
             RPCError::SSZDecodeError(_) => PeerAction::Fatal,
             RPCError::UnsupportedProtocol => {
@@ -359,6 +360,14 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
                 Protocol::Status => return,
             },
             RPCError::NegotiationTimeout => PeerAction::HighToleranceError,
+            RPCError::RateLimited => match protocol {
+                Protocol::Ping => PeerAction::MidToleranceError,
+                Protocol::BlocksByRange => PeerAction::HighToleranceError,
+                Protocol::BlocksByRoot => PeerAction::HighToleranceError,
+                Protocol::Goodbye => PeerAction::LowToleranceError,
+                Protocol::MetaData => PeerAction::LowToleranceError,
+                Protocol::Status => PeerAction::LowToleranceError,
+            },
         };
 
         self.report_peer(peer_id, peer_action);