SyncWriter and flush problem throwing Exception

When writing to topic code may produce a RuntimeException wraped in CompletionException.

For example, writing with codec = 10000 or 7.

 WriterSettings settings = WriterSettings.newBuilder()
                .setTopicPath(topicName)
                .setCodec(7)
                .build();
        SyncWriter writer = client.createSyncWriter(settings);
         writer.initAndWait();


        for (byte[] testMessage : testMessages) {
            writer.send(Message.newBuilder().setData(testMessage).build());
        }

        writer.flush();
        writer.shutdown(1, TimeUnit.MINUTES);

writer.flush(); - is the cause of our problem
Correct behavior—work without exception because we wait for all jobs to be completed and return results;

But if you machine is too quick - you will get CompletionException

After some research I can make test work more stable without throwing Exception(May be I was lucking). It's just a guess and should be checked:

1. WriterImpl - make lastAcceptedMessageFuture volatile. (Seems it's changed in different threads)
2. WriterImpl - acceptMessageIntoSendingQueue should return CompletableFuture
3. WriterImpl - tryToEnqueue should return acceptMessageIntoSendingQueue(message);

Start in WriterImpl at acceptMessageIntoSendingQueue, tryToEnqueue, and lastAcceptedMessageFuture, then trace Encoder.getCodec using the supplied codec 7 or 10000 reproduction. Run the flush scenario and verify that completion and failure behavior is deterministic and matches the issue's expected result.