IMPALA-7542: fix find-fragment-instances to find all "root threads"
IMPALA-7542: fix find-fragment-instances to find all "root threads"

find-fragment-instances didn't show all the threads
that worked on some fragment instance. It missed the
top-level "root threads" that started working on the
fragment instances.

I modified the get_fragment_instances() function to
check the local ThreadDebugInfo object of the threads
instead of checking the parent thread's ThreadDebugInfo.

I tested it locally on a core file.

Change-Id: I35ae1a6b384b002b343689469f02ceabd84af1b6
Reviewed-on: http://gerrit.cloudera.org:8080/11396
Reviewed-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>
Tested-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>

cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala