1.Fix primary writer gang leak: accidentally set PrimaryWriterGang to NULL which cause disconnectAndDestroyAllGangs()
  can not destroy primary writer gang.
2.Fix gang leak: when creating gang, if retry count exceed the limitation, forget to destroy the failed gang.
3.Remove duplicate sanity check before dispatchCommand().
4.Remove unnecessary error-out when a broken Gang is no longer needed.
5.Fix thread leak problem
6.Enhance error handling for cdbdisp_finishCommand