Reduce CacheEntry size

[adamsitnik]

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).

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.

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Issue Details

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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)

Author:	adamsitnik
Assignees:	-
Labels:	area-Extensions-Caching
Milestone:	6.0.0

[eerhardt]

I just had one minor question on the unit test. Other than that, I think this looks good.

[adamsitnik]

@eerhardt thanks a lot for the review!