do not use directExecutor which could be a gRPC thread

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java

@@ -1594,7 +1594,7 @@ public void run() {
  }
  }
  },
- MoreExecutors.directExecutor());
+ executor);
  return res;
  }