Improve JDBC repository session clean-up job

[vpavic]

See #838 for background (and more specifically this comment).

The approach with a single SQL statement based session clean-up job has been historically been somewhat problematic, seeing several revisions to address issues seen with specific RDMBSs. The currently used statement is susceptible to deadlocks in MySQL/MariaDB and SQL Server.

We should consider improving the session clean-up job so that it:

• is more robust, i.e. isn't susceptible to deadlocks
• provides more insight into expired sessions (debug logs etc.)
• doesn't execute on multiple nodes simultaneously in a cluster out-of-the-box
• is easier to customize