Frequent 'zkServer.sh stop' failures when running C test suite

As mentioned in https://github.com/apache/zookeeper/pull/1054#discussion_r314208678 :

There is a sleep 3 statement in zkServer.sh restart. I am unable to unearth the history of that particular line, but I believe part—if not all—of that sleep should be part of zkServer.sh stop.

I frequently observe FAILED TO START errors in the C test suite; the logs consistently show that those are caused by java.net.BindException: Address already in use. Adding a simple sleep 1 before echo STOPPED "fixes" it for me. I will submit an initial PR with the corresponding change and a commit message akin to:

ZOOKEEPER-XXXX: Make zkServer.sh stop more reliable

Kill is asynchronous, and without the sleep, the server's TCP port can still be busy when the next server is started—causing flaky runs of the C client's test suite.

(It would probably be better to spin a few times, probing with ps -p.)

As noted above, the sleep is far from optimal, an adaptive mechanism would be better—but I do not want to make the first iteration too complicated.