Flaky `zookeeper` feature tests

[mervyn-mccreight]

The tests of the feature zookeeper are very unstable. We need to fix this to get reliable feedback.

The output of a failing test example usually is:

running 1 test
  test zookeeper::tests::zookeeper_check_directories_existence ... FAILED
  
  failures:
  
  ---- zookeeper::tests::zookeeper_check_directories_existence stdout ----
  
  thread 'zookeeper::tests::zookeeper_check_directories_existence' (37197) panicked at src/zookeeper/mod.rs:102:83:
  called `Result::unwrap()` on an `Err` value: Error { source: None, message: "failed to enqueue new request: Canceled" }
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  
  
  failures:
      zookeeper::tests::zookeeper_check_directories_existence
  
  test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 12.01s