More changes for segment ownership, get rid of mutable segment()

[poodlewars]

A follow up to https://github.com/man-group/ArcticDB/pull/1756 .

This change is not essential and we should only merge it if we think it makes it easier to use the KeySegmentPair API correctly.

The non-const Segment& KeySegmentPair#segment() API on KeySegmentPair is a bit dangerous and error prone, because it isn't obvious that what it lets you modify is actually a resource that might be shared across several KeySegmentPair objects.

Instead we replace it with segment_ptr() and release_segment() APIs that return shared ptrs and unique ptrs, rather than hiding the ownership model from the KeySegmentPair API.