Fix for (mainly) test failures (use-after free)
I.e. test case test_commitlog_delete_when_over_disk_limit causes
use-after free because test shuts down before a pending flush is done,
and the segment manager is actually gone -> crash writing stats.
Now, we could make the stats a shared pointer, but we should never
allow an operation to outlive the segment_manager.
In normal op, we _almost_ guarantee this with the shutdown() call,
but technically, we could have a flush continuation trailing somewhere.
* Make sure we never delete segments from segment_manager until they are
fully flushed
* Make test disposal method "clear" be more defensive in flushing and
clearing out segments
05de462fa9