compaction: time_window_compaction_strategy: reset estimated_remaining_tasks when running out of candidates
compaction: time_window_compaction_strategy: reset estimated_remaining_tasks when running out of candidates

_estimated_remaining_tasks gets updated via get_next_non_expired_sstables ->
get_compaction_candidates, but otherwise if we return earlier from
get_sstables_for_compaction, it does not get updated and may go out of sync.

Refs #10418
(to be closed when the fix reaches branch-4.6)

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

Closes #10419

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