Add opt-in demotion database safety gate

tests/test-demote-database-gate.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT="$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)"
+FILE="$ROOT/ocf/loxthelion/managed-container.d/applications/postgresql/native_peers.sh"
+
+grep -q 'MC_DEMOTE_DATABASE_GATE' "$FILE"
+grep -q 'gate rejected demotion' "$FILE"
+grep -q 'if ! mc_pg_demote_database_shadow' "$FILE"
+
+echo "DEMOTE_DATABASE_GATE_TEST_PASS=1"