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/rpc/protocol.rs

@@ -421,6 +421,8 @@ pub enum RPCError {
     NegotiationTimeout,
     /// Handler rejected this request.
     HandlerRejected,
+    /// The request exceeds the rate limit.
+    RateLimited,
 }
 
 impl From<ssz::DecodeError> for RPCError {
@@ -459,6 +461,7 @@ impl std::fmt::Display for RPCError {
             RPCError::InternalError(ref err) => write!(f, "Internal error: {}", err),
             RPCError::NegotiationTimeout => write!(f, "Negotiation timeout"),
             RPCError::HandlerRejected => write!(f, "Handler rejected the request"),
+            RPCError::RateLimited => write!(f, "Request exceeds the rate limit"),
         }
     }
 }
@@ -477,6 +480,7 @@ impl std::error::Error for RPCError {
             RPCError::ErrorResponse(_, _) => None,
             RPCError::NegotiationTimeout => None,
             RPCError::HandlerRejected => None,
+            RPCError::RateLimited => None,
         }
     }
 }