The guard against "old segments" is pure slop. once upon a time we keyed mat updates by log server and guarded against stale segment updates (which have different log servers). When we changed to keying by cell, the clanker hallucinated some nonsense about "late arriving" segment updates, which in truth is a crash-worthy invalid state for mongo to be in, and we should not be trying to work around it here.

So remove the workaround. That leaves only the production possibility of a late arriving mat-update, which is fine because:

• If the update races the segment, then we are just advancing the mat update for the old segment like normal.
• if the segment races the update, either we wont have an update for that segment yet and its fine to process a mat update for the old segment in the middle, or there was an update for the new segment, and the old segment's update will be blocked, since it isn't monotonically increasing.