Pulsar: New schemas should overwrite deleted schemas

[rfuerst87]

Describe the bug
For what I observed deleting schemas currently just marks a schema deleted rather than actually delete it. Now for some reason schemas are simply ignored writing to a once-existing-but-now-deleted topic. Querying a schema for such a topic always returns a 404. The only workaround I found so far is to manually delete the schema in zookeeper.

To Reproduce

1. Write some messages to topic tenant/ns/topic using an AvroSchema for example
2. Query schema: pulsar-admin schemas get persistent://tenant/ns/topic -> Result ok
3. Delete topic including schema: pulsar-admin topics delete persistent://tenant/ns/topic -d
4. Again write messages to tenant/ns/topic with same AvroSchema as above
5. Query schema: pulsar-admin schemas get persistent://tenant/ns/topic -> Error 404 Not Found

Expected behavior
Deleted schemas should be overwritten by new schemas.

Desktop (please complete the following information):

• Pulsar version 2.4.0

Additional context
The same behavior can be observed when a topic is deleted automatically due to inactivity.

[congbobo184]

I hava already started to fix this bug :)