This commit refactors out the complexity of downloading ordered WAL
files in parallel to a type called `WALDownloader`. This makes it
easier to test the restore separately from the download.
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.
