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

[MikaelSmith]

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.

[MikaelSmith]

This appears to fix my reproduction case from #200.

[TwP]

This change makes perfect sense. I'm going to merge the fix and then tinker with the variable names a smidge - rename _reentrant_mutex to a simpler mutex.