Fix Logging::Logger#level= to be thread-safe
Fix Logging::Logger#level= to be thread-safe

Creating a new logger while also updating the level of an existing
logger can produce
```
can't add a new key into hash during iteration
```
because both operate on the same hash of loggers but use different
mutexes. Update them to share the same mutex.

Fixes #200.

TwP/logging,TwP/logging