coturn helm chart: use a memory-backed mount

[jschaul]

Add memory-backed mount /var/lib/coturn to store sqllite DB to improve performance, as described on https://github.com/wireapp/coturn/tree/master/docker/coturn#persistence

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[flokli]

We are certain we don't need any of that state?

[supersven]

@jschaul Well, I give you a conditional approval: If it doesn't turn out to be beneficial (no pun intended 😉 ), please roll the change back.

[supersven]

We are certain we don't need any of that state?

@flokli , @jschaul , AFAIK the state was previously saved in the container itself. That's not very persistent, too... 😉 However, maybe, @jschaul can try to understand what the database's data is about and if it would be beneficial to keep it in a persistent db?