Changed rollup stats call in Inspect2 to be single threaded to prevent deadlock (running out of user-mode threads) in case of dataset with many columns.

So the way our parallel rollupstats call works, if you have more than some number of columns in your dataset (hundreds) it will eat up all 99 low priority threads on just doing dfork on RollUpstats task and there will be no thread left to actually do the work and get some progress. For now, I changed the rollup stats call in Inspect 2 to do one column at a time.
Merge branch 'master' of github.com:0xdata/h2o