DEV: Change use of Redis flushall to flushdb.

FLUSHALL removes all keys from all databases. Instead we only want to
remove keys from the current Redis database.

spec/components/auth/default_current_user_provider_spec.rb

@@ -237,7 +237,7 @@ describe Auth::DefaultCurrentUserProvider do
     end
 
     after do
-      Discourse.redis.flushall
+      Discourse.redis.flushdb
     end
 
     it "should not update last seen for suspended users" do