ust-consumerd: fix exit race log corruption

ust-consumerd: fix exit race log corruption

In the following scenario on an SMP system the ust-consumerd can end
up not properly closing out file handles which leads to log
corruption:
  * usttrace -m -l small_quick_app_lots_of_malloc_and_free
  * The app completes and usttrace sees and sends the SIGTERM to ust-consumerd
  * The ust-consumerd main thread will exit and the _exit() handlers
    kills off the remaining pthreads without everything getting closed out

The solution to the problem is to introduce an active_thread count for
the private ustconsumer_instance.  This counter will be zeroed out
when it is safe to completely shutdown the main thread, which will
subsequently run the _exit() handlers.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>