SAMZA-2577 : Adding support for Async-Logger in Log4j2 Stream Appender

[rmatharu-zz]

Problem:
In both StreamAppender for log4j1 and log4j2 a blocking queue is used to coordinate between the append()-ing threads and a single thread send()-ing to Kafka.
This is a bounded, blocking, lock-synchronized queue.
To avoid deadlock scenarios (see SAMZA-1537), the append()-ing threads have a timeout of 2 seconds, after which the log message is discarded and the queue is drained.
This means in case of message bursts, threads calling append() may block for upto 2 seconds, and may continually be stuck in this pattern, leading to processing stalls and lowered throughput.

Solutions for Log4j2
Solution 1. Enable async logger in log4j2, since they are supported and provided in log4j2.https://logging.apache.org/log4j/2.x/manual/async.html.
In using this capability, the blocking-queue in StreamAppender is not required because the logger itself will be asynchronous, and so append() threads can directly call systemProducer.send().
However, if async loggers are not used then this queue based mechanism, to give the append()-ing threads an "async" illusion, is required.

Solution 2. Continue using the blocking bounded lock-based queue, but make the queue size and timeout configurable. Users can then tune this to account for message bursts.

Solution 3. Move to use a lock-less queue, e.g., ConcurrentLinkedQueue (unbounded) or
implement a bounded lock-less queue, or use [open-source implementations|https://stackoverflow.com/questions/20890554/lock-free-circular-array].
Append()-ing threads will no longer need to block or timeout. However the caller may busy-wait or need a fixed-rate or fixed-sleep-time to avoid busy waits, since a lock-less queue is non blocking.
It uses CAS operations.
For log4j2, we will adopt Solution 1.

[mynameborat]

Minor comments. LGTM;
Can we add unit tests for these?

[mynameborat]

Thanks for adding tests 👍