Initialize all memory for logical replication relation cache.
As reported by buildfarm animal skink / valgrind, some of the
variables weren't always initialized. To avoid further mishaps use
memset to ensure the entire entry is initialized.
Author: Petr Jelinek
Reported-By: Andres Freund
Discussion: https://postgr.es/m/
20170422183123.w2jgiuxtts7qrqaq@alap3.anarazel.de
Backpatch: none, code new in master