Add WARN when there are a lot of tombstones in a query

Now that we count the number of tombstones hit (so it can go in tracing), can we pick some threshold (or make it configurable with 0 being don't warn), and spit out a warning saying "Just went through 10000 tombstones in partition XYZ".

Right now if you are having GC problems because some row got a bunch of tombstones, you can turn on server side tracing, and hope the bad query gets in there, or you can keep making heap dumps, dig through them, and hope you catch the query in there.

I have seen code problems at multiple places causing this same issue (some code causing way more tombstones than it should, for just one row). And it is a PITA+Luck to debug it right now.