Potential concurrency problems in data path

[acloudiator]

There are some parts in some services that could have race conditions.

As of today, we have identified one in the nat (kubenat has it as well) [1]. It would be possible for that function to be invoked from more than a thread at the time, that will cause unexpected behavior.

I'm almost sure that there are similar issues in other services.

[1] https://github.com/polycube-network/polycube/blob/1f3dbfe9e01c415865425241be0939c7988cc4e5/src/services/pcn-nat/src/Nat_dp.c#L179