Change Memory Store to static

[JonPurvis]

Fixes saloonphp/saloon#392

This PR fixes a bug with the Memory Store which was causing all requests that run through it, to have their own store which was then causing rate limits to not work correctly. If you execute 50 requests, for example, then each of them creates a new MemoryStore instance.

The HasRateLimits trait caches the store per request:

public function rateLimitStore(): RateLimitStore
{
    return $this->rateLimitStore ??= $this->resolveRateLimitStore();
}

But when you do return new MemoryStore;, each request instance gets a different MemoryStore object, each with its own isolated $store array and because of this, they can't share the rate limit.

The solution is to change $store to a static property so all instances share the same storage. This makes the rate limit work across multiple request instances.

You can see below the rate limit code I had in my connector and then a before and after of my results running the exact same test:

protected function resolveLimits(): array
{
    return [
        Limit::allow(1)->everySeconds(1)->sleep()
    ];
}

protected function resolveRateLimitStore(): RateLimitStore
{
    return new MemoryStore;
}

Before	After