Represents a logical extent and its optional physical device starting offset.
Both logical_range boundaries and device_offset must always be a multiple of
BLOCK_SIZE (4096 bytes). The physical device range length is identical to logical_range.
Container for active mappings associated with a session.
Extents are stored in compact form (ExtentEntry, 16 bytes) in a boxed slice
sorted by ascending end_logical_offset to support clean O(log N) binary search lookups.