Not possible to use schema registry if a dependency uses it

[cottinisimone]

Since the Avrogen schema registry is essentially a global ETS table, it becomes awkward to use if you depend on avrogen in two different ways (say, directly, and via a dependency) because Avrogen.Schema.SchemaRegistry.init can't be called twice (you get Error: table name already exists).

The easiest fix is probably to make init check if the global table already exists with :ets.whereis, and add the current app's schemas to it if it does.

Also the schema registry doesn't really need to be a GenServer, it can just export the init_registry function that can be called from Application.start.

Created by @23Skidoo