Not registering different ids of the same avro entity

[danielwitz]

Describe the bug
When there are multiple ids to the same avro entity in the schema registry grabbit registers the first one (The one with the smaller id I think) and rejects the others as the entity is "already registered" in the map.
When an entity from the unregistered schema arrives grabbit fails the serialisation.

[rhinof]

@danielwitz WDYT should be the expected behavior in this case?

[danielwitz]

IMO, we should change the schemas map key to a schema name, schema id pair so in the case described both schemas are registered and when an object arrives with either of the schemas the serialization succeeds.
I think this is the more "whole" solution that ignores the type of the registry (backwards compatible, full transitive, none, etc).