Joblist thread pool leaks if mariadb client connection drops its connection early.

Joblist thread pool doesn't release threads occupied by JobList steps if mariadb client breaks its connection before the JobList step finishes.
To reproduce one should follow these steps:

• Joblist thread pool debugging must be enabled in Columnstore.xml first
  <JobList>
  <ThreadPoolDebug>Y</ThreadPoolDebug>
• run select * from tpch1c.lineitem and hit ctrl+c before the query returns the result set.
• then number of Active threads in
  /var/log/mariadb/columnstore/trace/ThreadPool_ExeMgrJobList.log must be >=
  10 and it won't decrease.