HETZNER: Enable cmode concurrent

[willpower232]

As per #3202, I believe Hetzner works with concurrency

[tlimoncelli]

CC @das7pad (provider owner)

Looking at the code, this should work fine.

If you'd like to verify empirically, please do the following:

1. Compile with the -race flag: go install -race
2. Run with a dnsconfig.js that has multiple domains from this provider (4 or more is preferable)
3. Run "dnscontrol preview --cmode concurrent` two or three times. You should get zero race condition errors

[tlimoncelli]

@das7pad ok with this change? I'd like to include this in the next release.

[das7pad]

Hi! Sorry for the delay in getting back to you. I've opened #3234 with the necessary changes to make the provider thread-safe.