Move support for DatasetRepositories that target key-value stores into cdk-data-core

Currently, the cdk-data-hbase module seems to include three distinct functions:

1) It maps an Avro schema to a key-value schema
2) It serializes/deserializes the attributes of entities using Avro and the schema mapping
3) It writes/reads the serialized bytes to HBase

We should move the first two functions into cdk-data-core and keep the cdk-data-hbase module focused on (3). That will also make it easier to support other key-value stores, like Accumulo, in the future.