refactor SharedMessagePool for easier control of allocation chunk size
remove most uses of MessagePool and make MessageBase smaller
Merge remote-tracking branch 'origin/nelson-mpi-communicator' into graphlabapi+mpi
add Process ID to 'frozen for debugger' message (each MPI version prints something different)
print metrics for graphlab/cc
fixup igor flags, define 'selected' results method
disable GRAPPA_FREEZE_ON_ERROR by default, set with grappa_srun flag