Clean up ZooKeeper group data on last member leave

[horkhe]

A consumer group data has never been removed from ZooKeeper before, that could cause issues with disposable groups that we use to implement broadcast events. With this PR when the last group member is leaving a group, its data structure in ZooKeeper is deleted. The logic is careful to survive a data race that can occur when the last member leaves but a new one joins the group at the same time.

Besides:

• dropped dependency on github.com/wvanbergen/kazoo-go in favor of an internal implementation that provides just what we need, makes fewer queries to ZooKeeper, and it is durable to data races;
• Fixed a long standing #54. Attempts to consume from a missing topics do not disrupt consumption from existing ones anymore.

[thrawn01]

I was unable to get kafka-pixy to clean up the /consumers in any situation.

• With 1 consumer connected, I shutdown kafka-pixy and /consumers zookeeper entry remains.
• With 1 consumer connected, I shutdown the consumer and wait for cleanup. /consumers zookeeper is never removed.

$ zkCli --servers localhost -c lsr /consumers                                                                                                                             │  -g, --group      consumer group we are in (Default=kafka-pixy-cli, Env=KAFKA_PIXY_GROUP)
scout                                                                                                                                                                     │  -b, --buffer     how many events to buffer before consumed (Default=0, Env=KAFKA_PIXY_BUFFER)
scout/ids                                                                                                                                                                 │
scout/owners                                                                                                                                                              │
scout/owners/catchall_legacy                                                                                                                                              │thrawn at Derricks-MacBook-Pro in ~
scout/owners/catchall_unicast                                                                                                                                             │$ kafka-pixy-cli consume catchall_legacy -g scout
scout/owners/scout_recount                                                                                                                                                │^[[A^C
scout_5938c8f68bb347db88e531db25b8b678                                                                                                                                    │
scout_5938c8f68bb347db88e531db25b8b678/ids                                                                                                                                │thrawn at Derricks-MacBook-Pro in ~
scout_5938c8f68bb347db88e531db25b8b678/owners                                                                                                                             │$ kafka-pixy-cli consume catchall_legacy -g scout -e localhost:19081
scout_5938c8f68bb347db88e531db25b8b678/owners/catchall_broadcast

Here is the log from when I started a single consumer and then stopped the consumer. https://gist.github.com/thrawn01/be1388b742efb47c33fe5838682dae74

I didn't have time to dig into why it wasn't cleaning up. If you want I should have more time tomorrow.

[horkhe]

@thrawn01 Looks like in your case subscriber stopped before partitioncsm released the partition. I wonder how respective test managed to pass. I will look into it tomorrow.

2018-10-16 14:07:19.705718 -05 error </_.0/cons.0/scout_5938c8f68bb347db88e531db25b8b678.0/member.0> "Failed to delete empty group" error="while deleting /consumers/scout_5938c8f68bb347db88e531db25b8b678/owners/catchall_broadcast: zk: node has children" kafka.group="scout_5938c8f68bb347db88e531db25b8b678"
2018-10-16 14:07:19.705753 -05 info </_.0/cons.0/scout_5938c8f68bb347db88e531db25b8b678.0/member.0> Stopped kafka.group="scout_5938c8f68bb347db88e531db25b8b678"
...
2018-10-16 14:07:19.708201 -05 info </_.0/cons.0/scout_5938c8f68bb347db88e531db25b8b678.0/catchall_broadcast.p0.0> "Partition released: via=/_.0/cons.0/scout_5938c8f68bb347db88e531db25b8b678.0/member.0, retries=0, took=2.02916ms" kafka.group="scout_5938c8f68bb347db88e531db25b8b678" kafka.partition=0 kafka.topic="catchall_broadcast"
2018-10-16 14:07:19.708226 -05 info </_.0/cons.0/scout_5938c8f68bb347db88e531db25b8b678.0/catchall_broadcast.p0.0> Stopped kafka.group="scout_5938c8f68bb347db88e531db25b8b678" kafka.partition=0 kafka.topic="catchall_broadcast"

[horkhe]

@thrawn01 fixed.