allow VM to talk to a co-locating server via a private load balancer

[byteocean]

This PR deals with a special communication scenario as depicted in the following figure (thanks to @guvenc). More precisely, due to the complex flow pattern, the connection tracking was not correctly performed and matched for packets. This PR includes two major changes:

1. move connection tracking related code out so as this tracking path can be reused;
2. correctly mark lb traffic as part of its flow key;
3. allow correct flow tracking and tcp state tracking for such scenario.

[guvenc]

@byteocean clang build fails.

[guvenc]

Some changes needed and some open questions. Thanks.

[guvenc]

@byteocean I pushed an improved version of the solution in this PR. Conntrack refactoring is ok but the way the traffic is isolated changed a bit. (More similar to the "other" uni-directional flows we have) Please take a look.
Please also remove the commit https://github.com/onmetal/net-dpservice/pull/341/commits/7dc87c27dfb802cd4ce090125e4297d2e130e983 from this PR, as it has its own PR.
Thanks.

[byteocean]

Some minor revisions on code comments have been added to be more clear in the code why it is written in this way, otherwise, it is a difficult special case that is hard to digest.

A improvement solution on the route instability issue (on-demand, which is different from #344 ) is also provided in this PR, otherwise, this PR cannot work properly.

[guvenc]

@byteocean What about age_ctx storage ? This still seems to be 4 ? Dont we need to increase it as discussed today ?

[byteocean]

@guvenc it was put into another branch, but now also fixed in this one

[guvenc]

@byteocean
Please remove the commit https://github.com/onmetal/net-dpservice/pull/341/commits/aa9947f30ba2082569e403506515f60fbd2d3d58 from this PR, as we discussed yesterday. It resolves a separate issue and I want to discuss it further in its own PR #344 . Please update that PR #344 with this new commit and remove it here so we can merge this one. Thanks.

[byteocean]

This new commit is removed. The point of having this commit is to make this PR a working one when route change is happening.