Add levels to planner debugger sanity checks

[alecgrieser]

There are some sanity checks that are expensive to execute (expensive enough that they can have deleterious effects on our PRB time) but can still be beneficial in debugging. For example, the traversal verification check ended up roughly doubling our total test execution time from 15 minutes to 30 minutes. It's been toned down to prevent the build from taking too long (see #3442), but if it finds anything, the developer would need to re-enable it at a higher level anyway.

An alternative would be to have a the sanity checks have certain levels à la logger levels. We could run those more expensive checks only at higher "sanity check levels" (or whatever name we give it).