GELF TCP output may block message indexing

When nothing is listening on the other side of a GELF TCP output, messages start queuing up and the whole server will stop indexing any messages given enough time.

@kroepke and I were looking at the related code and saw that the default timeout for each message is too high (10s). To mitigate the issue, the `output_module_timeout` configuration option may be set to a more sensible value.