#23 [17] Fix database issues

After implementation of the container handler / pipeline scheduler some concurrency issues regarding locked databse errors started to occur. To fix those issues a connection pool needs to be implemented. See: https://stackoverflow.com/questions/57123453/how-to-use-diesel-with-sqlite-connections-and-avoid-database-is-locked-type-of Another possible part of the solution (manual retries on DB errors) can be found here, but needs further investigation if indeed necessary: https://github.com/christian-oudard/sqlite-locking/blob/main/diesel/src/main.rs