load testing with concurrency crashes server

# Description
Running load testing with concurrency results in PyWPS crashing and not recovering.

# Environment

- operating system: Ubuntu 16.04.3
- Python version: 2.7.12
- PyWPS version: master
- source/distribution
- [x] git clone
- [ ] Debian
- [ ] PyPI
- [ ] zip/tar.gz
- [ ] other (please specify):
- web server
- [ ] Apache/mod_wsgi
- [ ] CGI
- [ ] other (please specify):

# Steps to Reproduce
- install PyWPS from master
- install PyWPS-flask from master
- install ApacheBench (ab)
- start `python demo.py`
- run `ab -c 10 -n 200 "http://localhost:9001/wps?service=wps&version=1.0.0&request=describeprocess&identifier=all"`, (`-c 10` means run 10 concurrent requests at a time) runs as expected and displays test results

`ab` will stop (Connection refused). In the server running window:

```
/home/ubuntu/pywps/local/lib/python2.7/site-packages/SQLAlchemy-1.2.8-py2.7-linux-x86_64.egg/sqlalchemy/util/langhelpers.py:74: SAWarning: An exception has occurred during handling of a previous exception. The previous exception is:
This transaction is closed
```

# Additional Information