Radix DRAM caching layer: accessing freed memory in case of OOM

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ISSUE: Radix DRAM caching layer: put/get/remove does not offer read commited guarantee in case of OOM

In heterogenous_radix bg thread inserts elements to radix tree in batch (in a single transaction). If inserting Nth element fails due to OOM all previous inserts will be rolled back. This means that pointers in dram cache will point to memory that is not allocated anymore.

This also violates read committed guarantees in some circumstances. When element is consumed by bg thread, cache entry for that element is marked as possible to evict. If such eviction happens and the bg thread fails due to OOM we lose that element.

One solution to this is to allow "partial commit" - committing operations that succeeded up to the point of OOM. This requires some changes in radix_tee: mainly, all operations which could throw OOM should be done at the beginning of the insert/erase so that the radix tree state stays consistent if we commit the transaction.