drill

813903a3 - DRILL-3779: Fix NPE in mergeAndSpill(). Add more debug logging messages. Make copier per-batch memory limit power of 2.

DRILL-3779: Fix NPE in mergeAndSpill(). Add more debug logging messages. Make copier per-batch memory limit power of 2. Get rid of some warnings. Add a few comments. Address review comments. Close apache/drill#160