Additional regression tests for CREATE OPERATOR.

Robins Tharakan, reviewed by Szymon Guz
Regression tests for LISTEN/NOTIFY/UNLISTEN/pg_notify.

Robins Tharakan, reviewed by Szymon Guz
Unbreak postmaster restart-after-crash sequence

In patch 82233ce7ea42, AbortStartTime wasn't being reset appropriately
after the restart sequence, causing subsequent iterations through
ServerLoop to malfunction.