fix negative rebalance estimated times
tweaked rebalance statistics calculations
Merge pull request #978 from VoltDB/fix_negative_rebalance_estimated_times

Fix negative rebalance estimated times
ENG-5543: Treat VoltAbortException in sysproc fragments as user error.
Merge pull request #981 from VoltDB/ENG-5543

ENG-5543: Treat VoltAbortException in sysproc fragments as user error.
Merge branch 'master' of github.com:VoltDB/voltdb into ENG-5486-null-xin