[logging] The Micropython Logging package implementation is not thread safe

CPython logging is thread-safe: [Logging Thread-safety doc](https://docs.python.org/2/library/logging.html#thread-safety)
The [Micropython logging package](https://github.com/pfalcon/pycopy-lib/tree/master/logging/logging) is not thread-safe, and this limitation is not mentioned in the Readme.