Fix possible crash reading pg_stat_activity.

With the old code, a backend that read pg_stat_activity without ever
having executed a parallel query might see a backend in the midst of
executing one waiting on a DSA LWLock, resulting in a crash.  The
solution is for backends to register the tranche at startup time, not
the first time a parallel query is executed.

Report by Andreas Seltenreich.  Patch by me, reviewed by Thomas Munro.