I'm a little worried that this whole question of changing the file naming scheme is a diversion which will result in torpedoing any chance of getting some kind of improvement here for v11. I don't think the patch is all that far from being committable but it's not going to get there if we start redesigning the world around it.
As stated above, the default 16MB has not changed and so we can take this separately and not as part of this patch.
PFA an updated patch which fixes a minor bug I found. It only increases the string size in pretty_wal_size function.
The 01-add-XLogSegmentOffset-macro.patch has also been rebased.
