Revised fix for ycmd terminate race condition

This PR fixes [issue #692](https://github.com/Valloric/YouCompleteMe/issues/692) by adding a few arguments to ycmnd:
```
--stdout <filename>: redirect stdout to <filename>
--stderr <filename>: redirect stderr to <filename>
--keep_logfiles: keep log files after the server quits
```

With this change, youcompleteme.py doesn't redirect ycmd.py's stdout/stderr, but just passes in arguments for ycmnd to do it itself.