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/methods.rs

@@ -258,6 +258,7 @@ pub enum RPCCodedResponse<T: EthSpec> {
 /// The code assigned to an erroneous `RPCResponse`.
 #[derive(Debug, Clone, Copy)]
 pub enum RPCResponseErrorCode {
+    RateLimited,
     InvalidRequest,
     ServerError,
     Unknown,
@@ -322,6 +323,7 @@ impl RPCResponseErrorCode {
             RPCResponseErrorCode::InvalidRequest => 1,
             RPCResponseErrorCode::ServerError => 2,
             RPCResponseErrorCode::Unknown => 255,
+            RPCResponseErrorCode::RateLimited => 128,
         }
     }
 }
@@ -332,6 +334,7 @@ impl std::fmt::Display for RPCResponseErrorCode {
             RPCResponseErrorCode::InvalidRequest => "The request was invalid",
             RPCResponseErrorCode::ServerError => "Server error occurred",
             RPCResponseErrorCode::Unknown => "Unknown error occurred",
+            RPCResponseErrorCode::RateLimited => "Rate limited",
         };
         f.write_str(repr)
     }