Improve NIOServerCnxn#isZKServerRunning to reflect the semantics correctly

This jira to improve the semantics of the following internal functions to make it more readable:

1. NIOServerCnxn#isZKServerRunning() => return true if the server is running, false otherwise.
2. AbstractFourLetterCommand#isZKServerRunning() => return true if the server is running, false otherwise.