ENH: Loading checkpoints

pb/comm.proto

src/libloom/fsutils.cpp

src/libloom/fsutils.h

src/libloomw/checkpointwriter.cpp

src/libloomw/worker.cpp

src/libloomw/worker.h

src/server/compstate.cpp

src/server/compstate.h

src/server/server.cpp

src/server/server.h

src/server/taskmanager.cpp

src/server/taskmanager.h

src/server/tasknode.cpp

src/server/tasknode.h

src/server/workerconn.cpp

src/server/workerconn.h

tests/client/test_checkpoint.py

tests/cpp/test_scheduler.cpp