ACCUMULO-2270 Make initialization of static threadpool thread-safe

Signed-off-by: Bill Havanki <bhavanki@cloudera.com>
ACCUMULO-2270 Make initialization of static threadpool thread-safe

Signed-off-by: Bill Havanki <bhavanki@cloudera.com>
Merge branch '1.5.2-SNAPSHOT' into 1.6.0-SNAPSHOT
ACCUMULO-2429 Add shell shutdown for thread cleanup

The JLine 2 ConsoleReader used by Shell spawns a thread which should be cleaned up when
done with the Shell. Otherwise, the thread leaks, taking up resources when the shell is
used programmatically. This commit adds a shutdown() method to Shell for cleaning up the
thread. This enables ShellServerIT to pass reliably and not flood the OS with leaked
threads.
ACCUMULO-2438 TabletGroupWatchers need to watch separate sets of tables for merges
Merge branch '1.6.0-SNAPSHOT' of https://git-wip-us.apache.org/repos/asf/accumulo into 1.6.0-SNAPSHOT