kvstore: use (*etcdClient).GetSessionLeaseID
kvstore: use (*etcdClient).GetSessionLeaseID

Use the existing method instead of open-coding it in
(*etcdClient).UpdateIfDifferentIfLocked and
(*etcdClient).UpdateIfDifferent.

Signed-off-by: Tobias Klauser <059b8b880f8441509ec8a65b50b4c6ae74ebea76@cilium.io>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium