Implement dpservice-ha synchronization

[PlagueCZ]

This PR implements a mechanism for synchonizing the active and standby instances of dpservice.

• synchronizes current NAT connections
• synchronizes current virtual service connections
• synchronizes VF neightbor MACs
• uses TAP devices in a bridge for synchronization
• connection tracking is not synchronized, but re-created when needed

Pytest suite and documentation are part of this PR

Fixes #697

Connected to #329

[PlagueCZ]

I am not sure I properly understand why test is failing, If I read it correctly, the problem is that the TAP devices and bridge were not properly created? Maybe the runner needs some privileges?

[PlagueCZ]

Solved. For some reason in a container pytest is not automatically refreshing interfaces. And when the test re-starts secondary dpservice, which uses the same interface names all the time, its interfaces are not properly registered in pytest.