Environment's held on longer than they need too

[maraisr]

When using useFragment instead of createFragmentContainer, in an SSR context the resourceMap holds onto the environment longer than it needs too, which causes the application to look like its got a memory leak, as the store is contained inside the environment. As touched on #2854 (comment)

I will prefix that with being, that in an SSR setting, you'd ideally want to blow away the store on every request, so you're not leaking store data between requests. So this only came about, when you create a new Environment, for the request. And having the resourceMap Map hold onto now discarded environments.

relay/packages/relay-experimental/FragmentResource.js

Lines 440 to 451 in 02d9e71

	const dataResources: Map<IEnvironment, FragmentResource> = new Map();
	function getFragmentResourceForEnvironment(
	environment: IEnvironment,
	): FragmentResourceImpl {
	const cached = dataResources.get(environment);
	if (cached) {
	return cached;
	}
	const newDataResource = createFragmentResource(environment);
	dataResources.set(environment, newDataResource);
	return newDataResource;
	}

My suggestion is to move away from a Map, and into a WeakMap, so that once the environment gets gc'd, it cleans up the fragment resourceCache.

- const dataResources: Map<IEnvironment, FragmentResource> = new Map();
+ const dataResources: WeakMap<IEnvironment, FragmentResource> = new WeakMap();