Sync only completed writes

Should be a final remaining fix to EC + non-capacitor (non-immediate-commit) write hangs :).

First it was breaking non-EC ("instantly stable") writes because they sometimes
complete out of order which was leading to the following error:

terminate called after throwing an instance of 'std::runtime_error'
  what():  BUG: Unexpected dirty_entry 1000000000001:29480000 v65540 unstable state during flush: 0x151

But it is easily fixed by scanning previous and next dirty_entries in mark_stable.

src/blockstore_rollback.cpp

@@ -162,6 +162,7 @@ void blockstore_impl_t::mark_rolled_back(const obj_ver_id & ov)
                     unstable_writes.erase(unstab_it);
                 else
                     unstab_it->second = max_unstable;
+                unstable_count_changed = true;
             }
         }
     }