Rework OVS tracking to properly support filtering

[amorenoz]

Now that filtering has been added and skb-tracking has moved to the core, OVS tracking has to be slightly modified if we want filtering to actually work.

There are currently two problems

• Userspace probes always run: there is not filtering.
• Action execute actions that come from upcalls are always run.

We need top propagate the filter through the user-space probes. But there is an added complexity: upcall_recv and flow_op events are correlated to each other based on their order (knowing how upcall batching works in OVS). Therefore, it's not enough to just ignore the event. We actually have to execute the batch-tracking but mark the packet as filtered so we don't send an event on it.

There are two places where we need tracking to happen: when we go from kernel to userspace (i.e: from upcall_enqueue event to upcall_recv event) and when we go from userspace back to kernel (i.e: from op_flow_exec event to execute_action event). For both, this PR adds a tracking map where we store each queue_id (i.e: the packet hash) as key and the timestamp as value. The reason for storing the timestamp is for garbage collection. It should be rare but it can happen that some events are missed and we end up with stale "packets" being tracked. A garbage collector is used to clean it.

Since a garbage collector also lives in skb-tracking.rs and we don't like code-duplication, it's refactored into a common utility struct and used in both places.

Based on #118

[amorenoz]

Rebased on top of master. Main changes:

• Adapted garbage collector to use Running object
• Added functional tests for OVS tracking
• Added a flag in the CI so that OVS tracking tests are only run on c9s and f38.
• Added python linting and formatting

Please not I've added an opinionated python format. It's quite used (and I really like it, it feels more "modern" than PEP8's) but please give me feedback on it: https://github.com/psf/black

[atenart]

This needs to be rebased to take into account commit 9844df7f0cc6.

[amorenoz]

Rebased on top of main with #165