checkpoint plugin logic fixes

This corrects some of the logic in
commit e8b1194841083c2c69b2382282a55586293d022d
There were several plugins storing the wrong plugin type code
(e.g. checkpoint/poe was recording state as checkpoint/blcr).
In addition, since there are new fields being stored, they
must be recorded as a different protocol_version, otherwise
all jobs would be lost when upgrading the system from an
earlier slurm release.