Currently, the moved-object's manager pointer is moved into the
constructed object, but without fixing the registration to
point to the moved-to object, causing #15248.
Although we could properly move the registration from
the moved-from object to the moved-to one, it is simpler
to just disallow moving a registered tracker, since it's
not needed anywhere. This way we just don't need to mess
with the trackers' registration.
With that in mind, when move-assigning a compaction_backlog_tracker
the existing tracker can remain registered.
Fixesscylladb/scylladb#15248
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>