Radix DRAM caching layer: improve layout of queue_entry

[igchor]

In heterogeneous_radix, uvalue_type wastes a lot of space for padding:

• padding between bool value and key_type (inline_string).
• padding between key and value (both inline_string). This is especially problematic for short keys (e.g. 8 bytes), because of the null-terminating character.