Revert "Move server.logger out of global scope"

This reverts commit 4a52b5145eb5a2c33cfe6a5a42f02451461a645f.

This code doesn't display anything:

>>> logger = logging.getLogger('x')
>>> logging.config.dictConfig(...)
>>> logger.info('hello world')

Unless 'disable_existing_loggers' is set to False in the dict config.

See https://docs.python.org/2/library/logging.config.html#logging.config.fileConfig

Consequently, there's no problem to setup the logger in the global scope
but disable_existing_loggers must be False, or server.py must be
imported *after* logging is configured.