ACCUMULO-1889 mark ZKI as closed once close() is called.

Once a given ZKI has .close() called, mark that instance closed regardless of outstanding client count.

Signed-off-by: Eric Newton <eric.newton@gmail.com>
ACCUMULO-1873 Better ZOOKEEPER_HOME check in start-all.sh

The script now checks that ZOOKEEPER_HOME is a directory and detects
when the search for a ZK JAR comes up empty.

Signed-off-by: Eric Newton <eric.newton@gmail.com>
ACCUMULO-1873 follow symlinks