Merge wait_slot_xmins() into get_slot_xmins().  At this point the only
place that wasn't doing a wait was the initial-state test, and a wait
there seems pretty harmless.

Michael Paquier

Discussion: https://postgr.es/m/CAB7nPqSp_SLQb2uU7am+sn4V3g1UKv8j3yZU385oAG1cG_BN9Q@mail.gmail.com