msal-node bug fix: acquireTokenSilent doesn't look in persistent cache

[shylasummers]

Fixed a bug in msal-node where acquireTokenSilent doesn't look in the persistent cache for a token if a valid token is not present in the in-memory cache.

• With the bug, if a valid token is not found in the in-memory cache, msal-node will make a network request
• With the fix, if a valid token is not present in the in-memory cache and a persistent cache is present, msal-node will overwrite the in-memory cache with the persistent cache and search for the token there before making a network request. This also keeps the in-memory cache in sync with the persistent cache