Re-check deprecation of setDefaultTopicConf

[nick-zh]

If seems for the low level consumer, some settings till need to be set over setDefaultTopicConf.
Check in librdkafka if we can get around using rd_kafka_conf_set_default_topic_conf which is said to be deprecated.

[nick-zh]

Got feedback from the maintainer of librdkafka:

You can set it on the global config object unless you pass a topic config object or a default topic config object.
In those two cases the global config for the topic will be ignored

I will check on our side if this happens somewhere, since it seems like the global config is ignored atm

[nick-zh]

So this is the current finding, after setting auto.offset.reset, a dump of the conf object will result in having:

...
    ['default_topic_conf'] = '0x55fd2d9260a0',
...

Which as mentioned above would cause a setting like auto.commit.interval.ms to be ignored on a global level.
Will further investigate if this happens inside librdkafka since i was unable to find code on our side that could cause this to happen.

[nick-zh]

auto.commit.interval.ms is a topic configuration for a low level consumer unlike the high level consumer where this is a global setting.
For libraries based on php-rdkafka or pure PHP implementations, this setting must be set on a topic config that can be passed to newTopic.
With this, i think it is fair to remove setDefaultTopicConf for the next major (was deprecated).

[Steveb-p]

@nick-zh we should remember to include this in upgrade path document, if it's created.

[nick-zh]

#387