Show reasoning graph URL on DEBUG level once

[bnouwt]

The reasoning graph visualization using GraphViz is quite useful and in the current master it was moved to TRACE level due to the log improvements. Maybe reintroduce it at DEBUG level, but not multiple times, but once before the reasoning starts.

[bnouwt]

Maybe also clean up TRACE logging a bit? Because now it contains a lot of stuff in which problems almost never occur. So, maybe remove those logging statements alltogether.