Add amcheck missing downlink tests.

Teodor Sigaev <teodor@sigaev.ru>

Commit: f35f30f74b6abb0a2f47c91c20df606cf166e8ff
Author: Teodor Sigaev <teodor@sigaev.ru>
Date: 2018-04-25T19:05:53Z
Releases: 11.0
Add amcheck missing downlink tests.

Also use palloc0() for main amcheck state, and adjust a few comments.

Somehow I pushed old version of patch in commit
4eaf7eaccb291f5d32d9f05284bfca9c5744de9a, so commit the difference.

Peter Geoghegan

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