couch_log: make sure that the the log file can be set in the ini

For some reason noone noticed until now, but changing the log file was
only possible in the app.config until now. This change fix this by doing
the following:

- when couch_log starts it check if the log file handler has already
  been started using the app.config. If not it start a new handler
- when the filename change after a config update using the API, the old
  file handler is shutdown and a new one is started with the new config.