fix race condition and some other things

[verygoodsoftwarenotvirus]

• fix race condition in FeatureFlagsPoller
• remove some useless whitespace, add some useful whitespace
• use errors.As in place of a type assertion
• stop shadowing the client struct in tests
• enable race detection in CI test runs

[neilkakkar]

Thanks for the contribution @verygoodsoftwarenotvirus :) - could you walk me through what's the race condition please, and why we need another mutex on top of the current one to fix it?

[verygoodsoftwarenotvirus]

could you walk me through what's the race condition please, and why we need another mutex on top of the current one to fix it?

I tried, and then I realized I couldn't, and in fact I could make use of the pre-existing mutex in a place where it was simply needed, and was able to satisfy the race detector anyway. Thanks.