Add error handling and retry to session_scope()

[nesnoj]

In 157e29a a temporary error handling was built into check_db_unique_violation() now covering UniqueViolation (IntegrityError), DeadlockDetected (OperationalError) with a retry functionality.

The "Too many clients already" #799 is not covered yet but could be. Alternatively, we could increase max_connections().

check_db_unique_violation() was not meant to handle DB errors in general, this could be generalized and moved to an appropriate place such as session_scope().

However, this would not cover

• queries that run outside the session scope (e.g. some SQL scripts),
• tasks that use sessionmaker instead of session scope (@ClaraBuettner knows about some..)

Having this background, does it make sense to you @gnn to integrate it into the session scope?

[nesnoj]

@gnn As agreed yesterday, the floor is all yours! :)