Hinted handoff bug

Created by: davecromberge

The hinted handoff process is triggered once a vnode enters an inactive state. This happens when the current active state times out, for a period of anywhere between 60s and 120s. basho/riak_core#715

A metric_vnode updates it's current time on a 1s tick, which is used to determine if a metric has a valid timestamp. This has the side effect of prolonging the active state, preventing the vnode from becoming idle.

Instead, the timestamp is now recreated on every invocation to the expiry function, allowing the tick message to be removed.