Enhance ResourceChecker to log stack trace for potentially hanging threads

Currently ResourceChecker logs a line similar to the following if it detects potential thread leak:

2012-11-02 10:18:59,299 INFO  [main] hbase.ResourceChecker(157): after master.cleaner.TestHFileCleaner#testTTLCleaner: 44 threads (was 43), 145 file descriptors (was 145). 0 connections,  -thread leak?-

We should enhance the log to include stack trace of the potentially hanging thread(s)

This work was motivated when I investigated test failure in HBASE-6796