…ptions

When an exception is thrown inside MemoryCache.GetOrCreate, we are leaking CacheEntry objects. This is because they are not being Disposed properly, and the async local CacheEntryStack is growing indefinitely.

The fix is to ensure the CacheEntry objects are disposed correctly. In order to do this, I set a flag to indicate whether the CacheEntry.Value has been set. If it hasn't, Disposing the CacheEntry won't add it to the cache.

Fix dotnet#42321

…inner cache's entries will reference the outer cache entries through the ScopeLease object.

Null'ing out the CacheEntry._scope field when it is disposed fixes this issue.