code: protected void waitOnDurabilityBackpressureFuture() {
          if (m_durabilityBackpressureFuture != null) {
              try {
                  m_durabilityBackpressureFuture.get();
              } catch (Throwable t) {
                  VoltDB.crashLocalVoltDB("Unexpected exception waiting for durability future", true, t);
              }
              durabilityTraceEnd();
          }
      }
docs: /*
  If async command logging is in use and the command log isn't keeping up
  an incomplete future will be populated here and can be waited on.
  
  Otherwise it will be null or will be populated with a completed future that
  returns immediately.