allow indexer block to lag behind latest RPC block

[frolic]

The sync stack waits for the current stored block to reach the latest RPC block before it's considered "live". However, when syncing from the indexer's streaming logs endpoint, the indexer will likely always be slightly behind the latest RPC block as it has the overhead of syncing itself, especially when under load. So the client has a hard time catching up to "live" and will often get stuck around 98-99% done.

Some options:

• consider the sync "live" once we switch to the live logs endpoint
• allow for some block lag between indexer block number and latest RPC block number
• do away with the notion of "live" and let apps decide if the app is caught up or not based on these two block numbers

[linear]

MUD-6 allow indexer block to lag behind latest RPC block

[frolic]

For now my workaround is

 export function Synced({ children, fallback }: Props) {
   const status = useSyncStatus();
-  return status.isLive ? children : fallback?.(status);
+  return status.latestBlockNumber > 0n &&
+    status.lastBlockNumberProcessed >= status.latestBlockNumber - 4n
+    ? children
+    : fallback?.(status);
 }