Update stopped flag to volitile in SystemCommandTasklet

The SystemCommandTasklet checks a flag to determine if another thread
has requested that it be stopped.  Since this flag will be set via
another thread than the executing thread, it needs to be declared
volitile so that it's state is propigated across threads correctly.  The
documentation for StoppableTasklet has also been upadted to call out
that scenario.

This commit addresses Jira BATCH-2271