Assert that the uv append index corresponds with the raft index.

[MathieuBordere]

I think we should do more checks within the code to assert that raft index of entries being appended to the log corresponds to the index that the uv backend is assigning to the entries. For this I think we need to add the raft index to struct raft_io_append in order to the comparisons. Just letting up a balloon here.