memberlist: Use advertised address when sending packets

[pstibrany]

Change TCP transport to use advertised address when sending "packets" (term used by memberlist lib). This was needed to stop memberlist complaining about not being able to ping other members of the cluster when running on internal Grafana k8s cluster.

(Memberlist is still experimental)

[bboreham]

Seems kinda nuts that we need a 600-line file just to add a transport to use a library.

I read your comments about mesh in the design doc; those 'todo's were written 2-3 years ago and it's extremely unlikely it will change except to fix bugs.

[pracucci]

I'm missing some knowledge about how how it works under the hood, but the changes in this PR look OK to me. I just left a tiny comment about commented code.

[pstibrany]

Seems kinda nuts that we need a 600-line file just to add a transport to use a library.

It would be nice if TCP-based transport was part of memberlist. There are some comments here and there indicating that people are using it with TCP transport, but nothing is included in the lib itself.

I read your comments about mesh in the design doc; those 'todo's were written 2-3 years ago and it's extremely unlikely it will change except to fix bugs.

If you want to encourage more people to use it (and maybe you don't, which is fine), it may be worthwhile to remove TODO's from the code, clean up the API little bit and add some documentation to it.

[jtlisi]

LGTM