Change Details

We currently don't check whether an existing table actually matches the schema specified in a table creation request, and just assume that it does. We should - check whether the schema matches (based on the json schema stored in the meta table, but possibly also based on the actual table schema as exposed by [system.schema_colums](https://github.com/wikimedia/restbase-cassandra/blob/221eee4272cacd133856b3aff33ebc96892c33d4/doc/Accounts_Buckets.md#table-naming-scheme--mapping-onto-cassandra). - if it doesn't, [see if we can do something about it](https://github.com/wikimedia/restbase/blob/master/doc/SchemaEvolution.md) - else, throw an error.