elasticsearch: fetch ganglia stats asynchronously

having ganglia callbacks block a long time makes gmond very sad, thus spawn a
(daemon) thread to fetch stats asynchronously. Having a daemon thread and not
clean it up on metric_cleanup() isn't the cleanest (ha ha!) approach but the
alternative is having a global variable to control when to stop.

Also note that the initial load to discover search groups is still blocking and
unlikely to be otherwise, but it happens only once at startup.

Change-Id: Ieb0367ffcfe0f33246c073cf4448a7ac16945620