because for fellow_cache_test there is no disk LRU, there is no instance to
trigger a log flush besides the periodic wake up of the logwatcher thread.

So when the disk buddy ran out of space, but the log buffer still had regions to
free, those would only be freed after two seconds, drastically slowing down
fellow_cache_test execution.

To avoid special casing just for tests, we now run a thread to care about this
issue specifically, to some extent emulating what otherwise the lru thread would
do.