[FEATURE] Memcached IOEngine

[VladRodionov]

Many applications:

• Local cache (client - side)-> Remote cache configuration support
• Instant cluster scale out, scale in support. Let us say we have cluster with 10 nodes (Config 1 includes 10 servers) and we want to add 5 more ( Config 2 with 15 servers). All 5 new servers will start with additional bottom tier - MemcachedIOEngine, which configured to use old Config 1 (10 servers). When new server has a miss it will access last engine in its hierarchy (get with timestamp is missing in memcached) retrieves it and saves locally. For this use case MemcachedIOEngine is temporal it will be shutdown once a certain metrics threshold is reached. For scale in the algorithm is pretty similar to the above one.

[VladRodionov]

This engine must support asynchronous reads, see #466