Avoid segment file name wrap around.

[kjnilsson]

Describe the bug

segment filenames are limited to 8 characters and are currently a base 10 number that is incremented every time a new segment is created. Although sufficient for most use cases it does mean that for very high throughput Ra clusters it is possible that the number will wrap around after a few years when creating a new segment every second or so.

A wrap around event would cause the segments to be sorted in the wrong order and would cause incorrect log reads.

Expected behavior

by adding a prefix to the segment name we can ensure it is much harder to cause a wrap around event.

Alternative 1: Add the expansion of "~YYMMDD" to the segment name as a prefix. This would allow the creation of over 1000 segment files per second which is extremely unlikely. This approach would also retain the sorting requirements even for servers with pre-existing segment files with the old format.

This alternative creates quite odd looking file names

Alternative 2:

upgrade to a 16 or 20 character name and rename existing segments. this can be done in the segment writer's init function and a marker file can be written to avoid re-entering the upgrade flow once done.

This also should be done for the WAL.