test_volume_io hang after several rounds of IO load and restarts (graceful)

[yamingk]

After several rounds of restarts, the HomeBlks/HomeStore will hang during IO load. This seems to be a cause of we can not sucessfully get a chunk select_chunk() during IO path, and it kept trying and hangs there.
If this is because of volume full, HomeBlks should be able to break out of the while loop and returning error because of no available_chunks. However this is unlikely the case, as from the logs it did 4 rounds of restarts and each round only do 300 IOs with mixed size.

homestore::cshared< Chunk > VolumeChunkSelector::select_chunk(homestore::blk_count_t nblks,
                                                              const homestore::blk_alloc_hints& hints) {
...
    do {
...
        LOGT("Waiting to allocate more chunks active={} total={}", volc->num_active_chunks.load(),
             volc->max_num_chunks);
        LOGT("{}", dump_chunks());
        std::this_thread::sleep_for(std::chrono::milliseconds(100)); // <<<< we are looping here
    } while (true);
...