CDAP-398 Add exists() and waitForX() to cdap-client classes

https://issues.cask.co/browse/CDAP-398
You're right, but this is due to a bug in my code. This was my intent: For example, if timeoutUnit is TimeUnit.MINUTES, then `timeoutUnit.toSeconds(1)` would be `1/60` and then in `ProgramFlowUtil.waitFor()` we'd sleep for `timeoutUnit.toMillis(sleepDelay = 1/60)` which would be 1000 milliseconds. But, it's actually just sleeping `sleepDelay` milliseconds. Will fix.