Ensure that Wayback index generation are fail safe

That is that indexes never are lost in the generation process in case of crashed. This should be at the expense of duplicate index entries. Eg. the sequence of index processed booking should be:

1. Process Index/File
2. Mark Process/Index as processed.

This means that in case of a crash between the 2 steps, the index step may be duplicated, but the index will always be processed at least one time.