We remove an LSM tree from the scheduler queues as soon as it is
dropped, even though the tree may hang around for a while after
that, e.g. because it is pinned by an iterator. As a result, once
an index is dropped, it won't be dumped anymore - its memory level
will simply disappear without a trace. This is okay for now, but
to implement snapshot iterators we must make sure that an index
will stay valid as long as there's an iterator that references it.

That said, let's delay removal of an index from the scheduler queues
until it is about to be destroyed.