SIFS.start() throws IllegalArgumentException: Negative position

This can happen when a record about deletion of entry with the same key was read recently and was not applied into index.

Fix is quite trivial, but this revealed a situation when an entry can be reincarnated during cache store startup - since we don't keep deleted entries in index, we can't see that an entry was deleted (don't know it's last seqId) if files are read in unlucky order.