Merge branch 'jm/cache-entry-from-mem-pool'

For a large tree, the index needs to hold many cache entries allocated on heap. These cache entries are now allocated out of a dedicated memory pool to amortize malloc(3) overhead. * jm/cache-entry-from-mem-pool: block alloc: add validations around cache_entry lifecyle block alloc: allocate cache entries from mem_pool mem-pool: fill out functionality mem-pool: add life cycle management functions mem-pool: only search head block for available space block alloc: add lifecycle APIs for cache_entry structs read-cache: teach make_cache_entry to take object_id read-cache: teach refresh_cache_entry to take istate