I've one more big improvement for the MemoryCache, but it's a little bit controversial so I decided to separate it from reducing the CacheEntry size (this PR).

Main changes:

• Remove _notifyCacheOfExpiration, _notifyCacheEntryCommit and _logger from CacheEntry, replace it with a reference to MemoryCache and instead of invoking delegates, invoke internal methods.
• Replace 3 boolean fields (_isDisposed, _isExpired and _valueHasBeenSet) with a single [Flag] field. I did this mostly because I plan to introduce another boolean flag soon.
• Remove _absoluteExpirationRelativeToNow and change the AbsoluteExpirationRelativeToNow property implementation to use AbsoluteExpiration property immediately. So far we were setting a private field and computing the actual value later in the SetEntry method.
• Don't acquire lock in DetachTokens if the field is already set to null
• avoid interface method call when the clock has not been provided (most common case)