Sync grafana/memberlist with upstream

[pracucci]

Mimir needs to use https://github.com/grafana/memberlist until hashicorp/memberlist#260 won't be reviewed and eventually merged. grafana/memberlist fork is lagging behind upstream. We should:

• Merge upstream into grafana/memberlist
• Upgrade grafana/memberlist in Mimir

[56quarters]

I'll pick this up if you haven't already started @pracucci