[CODE] Async operations support #100
Description
For tiered storage. Second tier is usually much slower than the first one. Executing fetch operation from a second tier asynchronously will improve performance overall. carrot cache introduced support for asynchronous operations by keeping a separate I/O thread pool for that purpose. This efficiently spilts threads into two pools: main - for fast operations in a first tier and I/O for a slower operations in a second tier.