This commit fixes an issue where the reference is taken
on the loop variable rather than the slice element when
computing the minimum snapshot within a generation so
it can cause the wrong snapshot to be chosen.
This commit changes the replica path format to group segments within
a single index in the same directory. This is to eventually add the
ability to seek to a record on file-based systems without having
to iterate over the records. The DB shadow WAL will also be changed
to this same format to support live replicas.
