Sqoop2: Add web interface for thread dump

Add web interface for thread dump, user can get the information from the shell.
the following is the example of output:

Process Thread Dump:
31 active threads

Thread 43 (Keep-Alive-Timer):
State: TIMED_WAITING
Blocked count: 0
Waited count: 1
Stack:
java.lang.Thread.sleep(Native Method)
sun.net.www.http.KeepAliveCache.run(KeepAliveCache.java:172)
java.lang.Thread.run(Thread.java:745)

Thread 42 (pool-1-thread-6 - /sqoop/threadDump?user.name=username):
State: RUNNABLE
Blocked count: 0
Waited count: 0
Stack:
sun.management.ThreadImpl.getThreadInfo1(Native Method)
sun.management.ThreadImpl.getThreadInfo(ThreadImpl.java:174)
sun.management.ThreadImpl.getThreadInfo(ThreadImpl.java:139)
org.apache.hadoop.util.ReflectionUtils.printThreadInfo(ReflectionUtils.java:165)
org.apache.sqoop.json.ThreadDumpBean.getThreadDump(ThreadDumpBean.java:44)
org.apache.sqoop.json.ThreadDumpBean.extract(ThreadDumpBean.java:37)
org.apache.sqoop.server.SqoopProtocolServlet.sendSuccessResponse(SqoopProtocolServlet.java:113)
org.apache.sqoop.server.SqoopProtocolServlet.doGet(SqoopProtocolServlet.java:52)