pkg/kvstore: add gRPC keep alives for etcd connectivity
pkg/kvstore: add gRPC keep alives for etcd connectivity

If the client does not receive a keep alive from the server, that
connection should be closed so the etcd client library does proper
round robin for the other available endpoints.

This might be a little bit aggressive in a larger environment if all
clients perform a keep alive requests to the etcd servers. Some
testing could be done to verify if there is a large overhead of
doing these keep alive requests.

Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

cilium-team/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,cilium-team/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium