Fix write->delete->write bugs, add & fix some debugging output

blockstore_stable.cpp

@@ -213,9 +213,6 @@ void blockstore_impl_t::mark_stable(const obj_ver_id & v)
                 break;
             }
         }
-#ifdef BLOCKSTORE_DEBUG
-        printf("enqueue_flush %lx:%lx v%lu\n", v.oid.inode, v.oid.stripe, v.version);
-#endif
         flusher->enqueue_flush(v);
     }
     auto unstab_it = unstable_writes.find(v.oid);