SetLogLevel defaultLevel not protected by mutex on wildcard case

I opened this PR (https://github.com/ipfs/go-log/pull/188) with an extra mutex protection for defaultLevel on wildcard case.

Also added a concurrency test that fails with the previous version when running with -race flag.

This was observed after upgrading our dependencies.

Start by reviewing pull request #188, which contains the proposed mutex protection and concurrency test. Run the test with the race detector to reproduce the previous failure; done means the wildcard case is race-free and the concurrency test passes.

Written by the indexing model from the issue text.