Async eviction

[vinser52]

This change is 

[victoria-mcgrath]

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cachelib/allocator/CacheAllocator-inl.h, line 1106 at r7 (raw file):

/* Next two methods are used to asynchronously move Item between memory tiers.
 *
 * The thread, which moves Item allocate, new Item in the tier we are moving to

Should comma go before "allocate"?

[victoria-mcgrath]

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cachelib/allocator/CacheAllocator-inl.h, line 1109 at r7 (raw file):

 * and calls moveRegularItemOnEviction() method. This method does the following:
 *  1. Create MoveCtx and put it to the movesMap.
 *  2. Updates the access container with the new item from the tier we are

"Update"? For consistency with the rest of the bullets.

[victoria-mcgrath]

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cachelib/allocator/CacheAllocator-inl.h, line 1113 at r7 (raw file):

 *  3. Copy data from the old Item to the new one.
 *  4. Unset thekNotReady flag and Notify MoveCtx
 * 

Extra spaces.

[victoria-mcgrath]

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cachelib/allocator/CacheAllocator-inl.h, line 1106 at r7 (raw file):

Previously, victoria-mcgrath wrote…

Should comma go before "allocate"?

"allocates"

[victoria-mcgrath]

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cachelib/allocator/CacheAllocator.h, line 1803 at r3 (raw file):

Previously, vinser52 (Sergei Vinogradov) wrote…

It was my initial idea. But folly::ConcurrentHashMap does not provide an interface that allows to modify mapped value in place. In our use case, we need to add waiters to the MoveCtx. But find method returns only a const iterator. There is `assign` method that allows to replace existing mapped value with the new one - it is not what we need as well.

Is extending an existing class an option?

[igchor]

Changes already merged into develop.