Review error result TTL handling in lookup caches

[kroepke]

When debugging a customer issue, the below code looks like it doesn't properly take the error result case into account.

graylog2-server/graylog2-server/src/main/java/org/graylog2/lookup/caches/CaffeineLookupCache.java

Lines 161 to 174 in d1fab26

	final LookupResult result = loader.call();
	if (ignoreResult(result, config.ignoreNull())) {
	LOG.trace("Ignoring failed lookup for key {}", key);
	return LookupResult.builder()
	.cacheTTL(0L)
	.build();
	}
	if (isResultEmpty(result)) {
	LOG.trace("Empty lookup for key {} with TTL {}", key, ttlEmptyMillis());
	return LookupResult.builder()
	.cacheTTL(ttlEmptyMillis())
	.build();
	}
	return result;

Error results have null values but are flagged as errors and come with custom TTL values. It looks like the code does not use that value and instead defaults to 0, "empty TTL" or forever.