Use send_after to reschedule stats refresh.
Use send_after to reschedule stats refresh.

To not accumulate refresh_stats messages if we're stuck.

Change-Id: I37fd39cbe9a33c8d95c1d9dde50bec20fa876c20
Reviewed-on: http://review.couchbase.org/38508
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

ceejatec/ns_server,mschoch/ns_server,mschoch/ns_server,membase/ns_server,membase/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,membase/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,nimishzynga/ns_server,mschoch/ns_server,ceejatec/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,membase/ns_server,membase/ns_server