Avoid serving validator endpoints while the node is far behind syncing head (#7962)

A performance issue was discovered when devnet-3 was under non-finality - some of the lighthouse nodes are "stuck" with syncing because of handling proposer duties HTTP requests.

These validator requests are higher priority than Status processing, and if they are taking a long time to process, the node won't be able to progress. What's worse is - under long period of non finality, the proposer duties calculation function tries to do state advance for a large number of [slots](d545ddcbc7/beacon_node/beacon_chain/src/beacon_proposer_cache.rs (L183)) here, causing the node to spend all its CPU time on a task that doesn't really help, e.g. the computed duties aren't useful if the node is 20000 slots behind.

To solve this issue, we use the `not_while_syncing` filter to prevent serving these requests, until the node is synced. This should allow the node to focus on sync under non-finality situations.

beacon_node/http_api/src/lib.rs

@@ -457,7 +457,7 @@ pub fn serve<T: BeaconChainTypes>(
                 move |network_globals: Arc<NetworkGlobals<T::EthSpec>>,
                       chain: Arc<BeaconChain<T>>| async move {
                     match *network_globals.sync_state.read() {
-                        SyncState::SyncingFinalized { .. } => {
+                        SyncState::SyncingFinalized { .. } | SyncState::SyncingHead { .. } => {
                             let head_slot = chain.canonical_head.cached_head().head_slot();
 
                             let current_slot =
@@ -479,9 +479,7 @@ pub fn serve<T: BeaconChainTypes>(
                                 )))
                             }
                         }
-                        SyncState::SyncingHead { .. }
-                        | SyncState::SyncTransition
-                        | SyncState::BackFillSyncing { .. } => Ok(()),
+                        SyncState::SyncTransition | SyncState::BackFillSyncing { .. } => Ok(()),
                         SyncState::Synced => Ok(()),
                         SyncState::Stalled => Ok(()),
                     }