When queries are slow on end-user machines it is hard to debug what the exact problem is.
The typical flow is to discover a "stuck" process. But since profiling logging was not enabled there is no output.
The user then has to re-run the problematic operation with profiling on, and locate the last output where the call gets stuck.
This PR introducces a monitor mechanism where all queries register themselves prior to execution, and then de-register once done.
At a fixed interval, the list of active queries is examined, and if a query is found to be running longer than the threshold, it will be logged as a warning.
This means the user can simply go view the live log for warnings, and the offending query will be emitted.
The default threshold is set at a conservative 30 minutes. Since the timer only checks at the interval, it can take at most 2x the interval before a message is logged. After this, a message is logged each time the interval is reached (e.g., a new warning is logged every 30 min).
The option `--long-database-query-threshold` can be used to set a different threshold.
