fixed handling of stats futures cancelling
fixed handling of stats futures cancelling

So that we can actually abort XHR when cancelling future.

Change-Id: I53e1b21967fb9663dc12b842341a340a5b1dd83d
Reviewed-on: http://review.membase.org/6174
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>
Tested-by: Dale Harvey <491ccac09a36ad1e38b698f273b84fd8efc31071@arandomurl.com>
Reviewed-by: Dale Harvey <491ccac09a36ad1e38b698f273b84fd8efc31071@arandomurl.com>

membase/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,vzasade/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,vzasade/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,membase/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,mschoch/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,mschoch/ns_server,vzasade/ns_server,mschoch/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,membase/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,ceejatec/ns_server,daverigby/ns_server,mschoch/ns_server