If we see an exception when adding or removing SSTables from the backlog
tracker, the backlog tracker can be inconsistent forever. It would be
best if we act before that happens and disable the backlog tracker. Once
the backlog tracker is disabled it will default to returning a fixed
number of shares.
We can either disable the backlog tracker or remove it. But if we remove
it we can end up with a backlog of zero if that's the only tracker with
a backlog. We then keep it registered but mark it as disabled. This also
leaves room for recovery in some situations: we can recover the backlog
by a doing a schema change in the column family that had the backlog
disabled, for instance.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
d4e7783188