Iglu Schema Registry: validate that schemas are uploaded to the correct path

[fblundun]

The "self" field of the schema should match the path to which it is uploaded.

[fblundun]

We could even make it possible to upload a schema either without the "self" field or without the path and have the server fill the missing information in automatically.

[alexanderdean]

Sounds good!

[chuwy]

Port implementation from Schema DDL.