The patch implements checkpoint support for multi gem5 runs. The checkpoint

must be initiated by a single gem5 process.  E.g., the gem5 process with rank 0

can take a checkpoint from the bootscript just before it invokes 'mpirun' to

launch an MPI test. The message server process will notify all the other peer

gem5 processes and make them take a checkpoint, too (after completing a global

synchronisation to ensure that there are no inflight messages among gem5).