feat: add max pending entries option to batch-processor

[Revolyssup]

Fixes #
When Kafka server is slow or unavailable, the logger objects keep accumulating faster than they are released causing memory spike.

The solution is to implement a limit in batch processor manager that will allow it to drop new objects pushed to it, if a given number of callbacks have not been processed.

Reproduction steps

1. Start APISIX (1 worker process), Kafka Server
2. Create a route and enable kafka-logger. Instance configuration:
   kafka-logger:
   batch_max_size: 80
   brokers:
   • host: 127.0.0.1
     port: 9092
     buffer_duration: 180
     cluster_name: 1
     inactive_timeout: 5
     kafka_topic: test2
     max_retry_count: 1
     meta_format: default
     meta_refresh_interval: 30
     name: service kafka logger
     producer_batch_num: 200
     producer_batch_size: 104857600
     producer_max_buffering: 50000
     producer_time_linger: 1
     producer_type: async
     required_acks: 1
     retry_delay: 1
     timeout: 5
3. Test the request with the curl command and make sure the request information is sent to the kafka server
4. Limit the CPU usage of the kafka server (simulating the slow receiving speed of the kafka server), for example, limit the CPU to 0.1 CUP
   sudo docker update --cpus 0.1 apisix_kafka
5. Use wrk/wrk2 to send pressure to the specified route, and you will soon see the memory usage of the APISIX worker increase rapidly. When it grows to a certain size, it will be killed by the system, causing the worker process to crash and exit , and then APISIX will start a new worker process.
   wrk2 -c 200 -d 60 -t 4 -R 50000 http://xxxxx/test

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[nic-6443]

please read the above code, for whole not ok case, the failed batch will be discard after exceed max_retry_count, in this case, batch will be free too, so it should be counting in processed_entries too.

[nic-6443]

can we add test cases for this feature? start a mocking tcp server that don't read data from connected socket, this will accumulating pending log entry in apisix, and we can set max_pending_entries to small value to confirm discard code is working.

[membphis]

In this pr, I think we only need to update one plugin kafka-logger, this should be enough

We can change other plugins later via separate PRs

[membphis]

need to print more useful information, eg: max_pending_entries, self.total_pushed_entries - total_processed_entries(self)

[membphis]

bad indentation

[membphis]

ditto

[membphis]

the minimum should be 1 here

0 is meaningless for APISIX user.

[membphis]

LGTM, except one minor issue