Conversion of checkpoints in iss.go

[matejpavlovic]

The checkpoints handled in iss.go are sometimes converted back and forth between their generated types and the underlying protobufs. These conversions look quite meaningless. Check where there is really no purpose to those conversions and remove them if possible.

[matejpavlovic]

This is, actually, just an awkward way of converting a mir-generated checkpoint type to the "native" checkpoint type through a "raw" protobuf. What needs to be cleaned up is loading checkpoints directly from the mir-generated types, not from the raw protobuf.