reset msg_counter when checkpoint is called, add 'rewind' method.
reset msg_counter when checkpoint is called, add 'rewind' method.


Former-commit-id: 4073aaed7134387bb1766e80ec07b1517b2784e0
jswhit/py-ncepbufr,jswhit/py-ncepbufr,jswhit/py-ncepbufr