bptree: Require dynamic object for nodes reconstruct
bptree: Require dynamic object for nodes reconstruct

The B+ tree is not intrusive and supports both kinds of objects --
dynamic (in sense of previous patch) and fixed-size. Respectively,
the nodes provide .storage_size() method and get the embedded object
storage size themselves. Thus, if a dynamic object is used with the
tree but it misses the .storage_size() itself this would come
unnoticed. Fortunately, dynamic objects use the .reconstruct()
method, so the method should be equipeed with the DybnamicObject
concept.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla