Test & fix single-PG primary OSD

- Add support for benchmarking single primary OSD in fio_sec_osd
- Do not wait for the next event in flushers (return resume_0 back)
- Fix flushing of zero-length writes
- Print PG object count when peering
- Print journal free space when starting and when congested

blockstore_flush.h

@@ -45,7 +45,7 @@ class journal_flusher_co
     std::map<object_id, uint64_t>::iterator repeat_it;
     std::function<void(ring_data_t*)> simple_callback_r, simple_callback_w;
 
-    bool skip_copy, has_delete;
+    bool skip_copy, has_delete, has_empty;
     spp::sparse_hash_map<object_id, clean_entry>::iterator clean_it;
     std::vector<copy_buffer_t> v;
     std::vector<copy_buffer_t>::iterator it;