Add asynchronous touch option for max-idle

When an entry has max-idle expiration, the key owner processing a get operation delays the response to send touch RPC to all the other owners of the key and to receive their responses. This guarantees to the application that if the get operation was successful, the entry is not going stay alive for another max-idle interval.

Many applications do not need this strong guarantee, and can tolerate a small number of entries expiring too soon when a node crashes in exchange for a lower latency on get operations.

We should add an option to send the touch RPC asynchronously in DIST_SYNC/REPL_SYNC caches. In DIST_ASYNC/REPL_ASYNC caches we should always send the touch RPC asynchronously.