model-server/zookeeper_cc: improve tests
model-server/zookeeper_cc: improve tests

* Make `StartZookeeper()` wait until connection is re-established by polling
  `State()` method for some time.
* Add tests for `Exists` watcher:
  * Set watch, create/remove node, receive event.
  * Set watch, restart server, create node, receive event.
  * Stop server, try to set watch, start server, ensure that watch is not set.

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