Pointer to Segment in SegmentAccessor might expire

[mrks]

SegmentAccessor::_segment is just a reference. In some cases, we seem to create accessors and let the shared_ptr to the segment expire. This can lead to the segment being deleted while the accessor is still existant.

Either fix those cases and make sure that the shared_ptr is held for long enough (might be difficult with ReferenceSegments) or simply make _segment a shared_ptr (and check the performance impact).