Increase maximum gossipsub subscriptions (#2531)

Due to the altair fork, in principle we can now subscribe to up to 148 topics. This bypasses our original limit and we can end up rejecting subscriptions. 

This PR increases the limit to account for the fork.

beacon_node/eth2_libp2p/src/behaviour/mod.rs

@@ -216,8 +216,8 @@ impl<TSpec: EthSpec> Behaviour<TSpec> {
                 chain_spec.attestation_subnet_count,
                 SYNC_COMMITTEE_SUBNET_COUNT,
             ),
-            max_subscribed_topics: 200, //TODO change this to a constant
-            max_subscriptions_per_request: 100, //this is according to the current go implementation
+            max_subscribed_topics: 200,
+            max_subscriptions_per_request: 150, // 148 in theory = (64 attestation + 4 sync committee + 6 core topics) * 2
         };
 
         config.gs_config = gossipsub_config(fork_context.clone());