Handle random db disconnect 
Handle random db disconnect 

while testing my endpoints I found some random errors due to a DB disconnect issue,  -  reading the logs:  https://gist.github.com/epifanio/9970069e38457db1fa9ad248ec442ffa    the sqlalchemy docs have some hints on how to deal with such errors. I found that adding  `pool_pre_ping=True` in  the  `sqlalchemy.create_engine()` call in https://github.com/geopython/pycsw/blob/master/pycsw/core/repository.py#L74  - seems to help -  the random error is gone (so far)
ricardogsilva/pycsw,ricardogsilva/pycsw,ricardogsilva/pycsw,kalxas/pycsw,geopython/pycsw,geopython/pycsw,kalxas/pycsw,geopython/pycsw,tomkralidis/pycsw,tomkralidis/pycsw,bukun/pycsw,tomkralidis/pycsw,bukun/pycsw,bukun/pycsw,kalxas/pycsw