Increased session timeouts for ZooKeeper related tests.

- On slower machines sometimes the zookeeper c client times out where we aren't expecting because either the test server or the client is too slow to respond. Increasing this value helps mitigate the problem.

Review: https://reviews.apache.org/r/25487
Pulled the log line in ZooKeeperTestServer::shutdownNetwork() to above the shutdown call.
- When debugging zookeeper related tests it's often more useful to know when the tests is shutting down the ZK server to reason about the order of events. Otherwise client disconnections are often logged before this "shutdown" line and can be confusing.

Review: https://reviews.apache.org/r/25511