Bound the time that stats calculation can take

add a timeout to stat calculation so that stuck or extremely slow processes no longer keep stats from proceeding forever.

cc @jonmeredith @russelldb 

Note that jon already did a revew, so we may just want to merge this.

We took two different approaches to testing this: 

- I mecked riak_kv_vnode:vnode_status to never return (which is what we were seeing in the field) and instrumented the code to be more verbose in logging.   
- Jon IIRC set the proxy overload length to 1 and then loaded the cluster to get a more "natural" reproduction.

This should also get into develop, along with a fix for the base problem (vnode_status not being overload safe).