Add JDK 6 version of ServerInfo.listThreadDump() taking advantage of new management classes

There're 4 ways to generate a thread dump. I have taken the output from the same run with
different methods:

1.- kill -3. Can be parsed by tools like https://tda.dev.java.net/ (simply, one of the most useful
tools I've found out there!). Example:

"JBossLifeThread" prio=1 tid=0x0a690cc0 nid=0x1741 in Object.wait() [0x867f2000..0x867f30b0]
at java.lang.Object.wait(Native Method)

• waiting on <0x9247e200> (a java.lang.Object)
  at java.lang.Object.wait(Object.java:474)
  at org.jboss.system.server.ServerImpl$LifeThread.run(ServerImpl.java:940)
• locked <0x9247e200> (a java.lang.Object)

2.- JMX via org.jboss.system.server.ServerInfo.listThreadDump(). Here's some output:

Thread: JBossLifeThread : priority:5, demon:false, threadId:58, threadState:WAITING, lockName:java.l
ang.Object@10cff6b

java.lang.Object.wait(Native Method)
java.lang.Object.wait(Object.java:474)
org.jboss.system.server.ServerImpl$LifeThread.run(ServerImpl.java:940)

Lock name is different to lock monitor achieved via kill -3. Doesn't looks like it prints previously locked
monitors. Cannot be parsed by TDA tools because it's in JBoss specific format.

3.- Thread.getAllStackTraces().

Haven't tested the output from this. What would this print? Couldn't find an MBean that uses/parses
the output from this, but I suspect it's the same as method 2, specially since
http://jira.jboss.com/jira/browse/JBAS-1448 has already been fixed.

4.- JConsole Threads view:

Code:

Name: JBossLifeThread
State: WAITING on java.lang.Object@10cff6b
Total blocked: 0 Total waited: 1

Stack trace:
java.lang.Object.wait(Native Method)
java.lang.Object.wait(Object.java:474)
org.jboss.system.server.ServerImpl$LifeThread.run(ServerImpl.java:940)

Seems to print same info as JBoss' ServerInfo.

Bottom line: we should try generating thread dumps that look like kill -3
output to make them more readable than in their current format and to
benefit from existing tools that can parse standard kill -3 format.