vinyl: simplify range index file naming

Currently, we name range index file by the lsn from the time of the last
checkpoint. As a result, each time we checkpoint an index we have to
relink its range index file. Actually, all we want to achieve here is to
have different range index files for different reincarnations of the
same index. This can be done in an easier way - we can just name range
index file by the lsn of the index creation, which was added to key_opts
by the previous patch. This way we don't have to rename any files.