apps: memcached: reduce the error during the expiration time translation
apps: memcached: reduce the error during the expiration time translation

Durations were rounded to seconds twice when a delta between a
wall clock and a monotonic clock was calculated, which allowed up
to near 2 seconds error. Reduce an error by changing a delta
counter to hold clock_type::duration value instead of seconds.

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>
Message-Id: <1452012791-4410-2-git-send-email-976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>

kjniemi/seastar,scylladb/seastar,avikivity/seastar,kjniemi/seastar,dreamsxin/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,sjperkins/seastar,sjperkins/seastar,avikivity/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,kjniemi/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,sjperkins/seastar