fix(backend): memory leak in memory caches (#14363)

* encapsulate `MemoryKVCache<T>`

* remove infinity caches

* encapsulate other caches

* add missing awaits to internally synchronize caches

* implement pull-through caching

* tune cache lifetimes

* optimize cache GC by stopping early

* summarize changes in CHANGELOG.md

* Fix timeout comments

Co-authored-by: かっこかり <67428053+kakkokari-gtyih@users.noreply.github.com>

* add comments about awaiting the redis write

---------

Co-authored-by: かっこかり <67428053+kakkokari-gtyih@users.noreply.github.com>

packages/backend/src/core/RelayService.ts

@@ -35,7 +35,7 @@ export class RelayService {
 		private createSystemUserService: CreateSystemUserService,
 		private apRendererService: ApRendererService,
 	) {
-		this.relaysCache = new MemorySingleCache<MiRelay[]>(1000 * 60 * 10);
+		this.relaysCache = new MemorySingleCache<MiRelay[]>(1000 * 60 * 10); // 10m
 	}
 
 	@bindThis