guard all access to item.promotions
2 changes:

1 -
Previously, we determined if an item should be promoted in the main getter
thread. This required that we protect the item.promotions variable, as both
the getter and the worker were concurrently accessing it. This change pushes
the conditional promotion to the worker (from the getter's point of view, items
are always promoted). Since only the worker ever accesses .promotions, we no
longer must protect access to it.

2 -
The total size of the cache was being maintained by both the worker thread
and the calling code. This required that we protect access to cache.size. Now,
only the worker ever changes the size. While this simplifies much of the code,
it means that we can't easily replace an item (replacement either via Set or
Replace). A replcement now involves creating a new object and deleting the old
one (using the existing deletables and promotable infrastructure). The only
noticeable impact frmo this change is that, despite previous documentation,
Replace WILL cause the item to be promoted (but it still only does so if it
exists and it still doesn't extend the original TTL).