Producer can't produce messages with an old schema version.

[codelipenghui]

Describe the bug

Producer can't produce messages with an old schema version.

To Reproduce
Steps to reproduce the behavior:

1. Use pulsar-admin to set-schema-autoupdate-strategy -c Backward public/default
2. Run following test case:

import org.apache.pulsar.client.api.PulsarClient;
import org.apache.pulsar.client.api.PulsarClientException;
import org.apache.pulsar.client.api.Schema;

public class SchemaTests {

    public static void main(String[] args) throws PulsarClientException {

        PulsarClient client = PulsarClient.builder().serviceUrl("pulsar://127.0.0.1:6650").build();
        client.newProducer(Schema.AVRO(UserV1.class)).topic("topic-1").create();
        client.newProducer(Schema.AVRO(UserV2.class)).topic("topic-1").create();
        client.newProducer(Schema.AVRO(UserV1.class)).topic("topic-1").create();

    }


    private static class UserV1 {

        private String username;
        private int age;

        public String getUsername() {
            return username;
        }

        public void setUsername(String username) {
            this.username = username;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }
    }

    private static class UserV2 {

        public String getUsername() {
            return username;
        }

        public void setUsername(String username) {
            this.username = username;
        }

        private String username;

    }

}

Will produce the following error:

Exception in thread "main" org.apache.pulsar.client.api.PulsarClientException$IncompatibleSchemaException: org.apache.pulsar.broker.service.schema.exceptions.IncompatibleSchemaException: Incompatible schema used
	at org.apache.pulsar.client.api.PulsarClientException.unwrap(PulsarClientException.java:253)
	at org.apache.pulsar.client.impl.ProducerBuilderImpl.create(ProducerBuilderImpl.java:88)
	at org.apache.pulsar.issue.SchemaTests.main(SchemaTests.java:12)

Expected behavior

At producer side, producer can send messages using an old version schema.

[codelipenghui]

@congbobo184 please take a look this issue.

[jiazhai]

would like to move this to 2.5.0

[sijie]

This has been addressed by #5227