Prefetch Improvement

[dboehm-avalabs]

Improve Prefetch by not taking the cache write lock for items already present in the cache.

[patrick-ogrady]

In practice, this was shown to greatly improve concurrent prefetch performance (reads don't need to acquire the write lock on the cache like Put).

I'm wondering if it would make sense to include this optimization directly in the cache vs here or if this is considered non-standard usage/optimization? @StephenButtolph

[StephenButtolph]

I'm not sure we can directly include this into the cache because the cache would then need to somehow check if the provided value is equal. We could do that just directly with ==, but I feel like if we wanted to do that then we would want to make it more general.

[danlaine]

comment nit, otherwise lgtm