The problem was a race condition on shutdown -- when IM got shutdown
request while a guarded mysqld is starting. In this case the Guardian
thread tried to stop the mysqld, but might fail if the mysqld hadn't
created pid-file so far. When this happened, the mysqld-monitor thread
didn't stop, so the assert in Thread_registry happened.

The fix is to make several attempts to stop mysqld if it is active.


server-tools/instance-manager/guardian.cc:
  Try to stop mysqld several times if it is still active.
server-tools/instance-manager/instance.cc:
  Make Instance::kill_mysqld() to return operation status.
server-tools/instance-manager/instance.h:
  Make Instance::kill_mysqld() to return operation status.
server-tools/instance-manager/thread_registry.cc:
  Log unregistered thread ids.