fix: prevent race condition when updating rate limit data

Previously if you made two requests simultaneously there was a chance
that the rate limit data would get messed up. Specifically, consider the
following scenario:

1. A new rate limit window starts, with 100 requests remaining.
1. Two requests are made (A and B)
2. Request A is completed first, so now there are 99 requests left, but
   is delayed in the network
3. Request B completes second, reducing the remaining count to 98, but
   takes a faster path back
4. Response B is processed first, and the rate limit data is set to 98
   remaining.
5. Response A is processed, and the rate limit is updated to 99 left.

Clearly this is incorrect. This is a very minor inconsistency, as the
next request will overwrite the value, but it still has the potential to
confuse any algorithms that expect the value to monotonically decrease
during a period.

src/gateway/gateway.ts

@@ -216,12 +216,24 @@ export abstract class Gateway {
     const { headers } = response;
     const remain = Number.parseInt(headers["x-ratelimit-remaining"] as string);
     const reset = Number.parseInt(headers["x-ratelimit-reset"] as string);
-    this.rateLimit = { remaining: remain, reset: Date.now() + reset * 1000 };
-    debug.general(
-      "Updated ratelimit: %d requests remaining, resets at %s",
-      this.rateLimit.remaining,
-      new Date(this.rateLimit.reset)
-    );
+
+    // To prevent race conditions, only update the rate limit if either...
+    if (
+      // ...we don't have one stored
+      !this.rateLimit ||
+      // ...the stored data is expired
+      this.rateLimit.reset < Date.now() ||
+      // ...the number of remaining requests has decreased
+      this.rateLimit.remaining > remain
+    ) {
+      this.rateLimit = { remaining: remain, reset: Date.now() + reset * 1000 };
+      debug.general(
+        "Updated ratelimit: %d requests remaining, resets at %s",
+        this.rateLimit.remaining,
+        new Date(this.rateLimit.reset)
+      );
+    }
+
     return response;
   }
 }