store: fix data race when modify event in watchHub.

[xiang90]

The event got from watchHub should be considered as readonly.
To modify it, we first need to get a clone of it or there might
be a data race.

Fix kubernetes/kubernetes#18437

/cc @gyuho @wojtek-t @timothysc

[wojtek-t]

Great - thanks @xiang90

Would it be possible to patch it to 2.2.2 release once this is merged?

[xiang90]

@wojtek-t Probably early next week.

[gyuho]

LGTM.

[wojtek-t]

@wojtek-t Probably early next week.

thanks - can you please ping me then - I will update our dependency on etcd in k8s

[xiang90]

@wojtek-t sure.

[wojtek-t]

@yichengq @xiang90 - can we have this merged?

[wojtek-t]

@xiang90 - thanks a lot!

[timothysc]

@xiang90 could we get this change on a 2-series branch, I'm updating deps right now.

[wojtek-t]

@xiang90 @yichengq - when can we expect this PR patched in 2.2 branch?

[xiang90]

@wojtek-t I will do the cherry picks today. And release tomorrow hopefully

[wojtek-t]

@xiang90 - thanks a lot!

I guess release is not that urgent, since we have problems in unit tests, and for those we simply have etcd in our Godeps (for which I guess we don't need an official release).

[xiang90]

@wojtek-t @timothysc

Cherry picked to https://github.com/coreos/etcd/tree/release-2.2 branch

[wojtek-t]

@xiang90 - thanks a lot for help!