fix(endpoint-cache): reduce bundle size (#2400)

mnemonist is a pretty big commonjs package, which is currently included
IN FULL from the endpoint-cache package. This is less than ideal when
using the sdk on the frontend side, since it adds about 270kb to the
bundle (before minification).

Since only the LRU-Cache is needed, changing the import to directly
target the lru-cache module helps bundlers tree shake correctly,
reducing the included code to about 15kb.

packages/endpoint-cache/src/EndpointCache.spec.ts

@@ -1,9 +1,9 @@
-import { LRUCache } from "mnemonist";
+import LRUCache from "mnemonist/lru-cache";
 
 import { Endpoint } from "./Endpoint";
 import { EndpointCache } from "./EndpointCache";
 
-jest.mock("mnemonist");
+jest.mock("mnemonist/lru-cache");
 
 describe(EndpointCache.name, () => {
   let endpointCache;

packages/endpoint-cache/src/EndpointCache.ts

@@ -1,4 +1,4 @@
-import { LRUCache } from "mnemonist";
+import LRUCache from "mnemonist/lru-cache";
 
 import { Endpoint } from "./Endpoint";