Major refactoring of the SchemaManager (previously known as EntityManager) subsystem.
The SchemaManager no longer creates entity mappers. It now only provides schema
metadata querying, schema creation, and schema migration.

The schema management is now in a package called com.cloudera.cdk.data.hbase.manager

The sole implementation of the SchemaManager interface, DefaultSchemaManager, is not
tied to any particular schema parsing implementation. In fact, it will construct
the appropriate schema parser indicated by metadata on the managed schema record.

It's Dao implementations responsibility to construct entity mappers from the
SchemaManager's schema values. The Avro dao implementations construct a new
VersionedAvroEntityMapper which has the ability to read rows of varying schema
versions.

The ManagedSchema record now contains metadata about the schema parser to use,
and the key and entity serdes rows are persisted with. The SchemaManager uses
the schema parser metadata to construct the appropriate schema parser. It's the
Dao's responsiblity to construct the the appropriate key and entity SerDe
implementations indicated in the ManagedSchema record. The current Avro
implementation doesn't do this today since there is only one, but it's trivial
to add this in when needed.

EntitySchema implementations, like AvroEntitySchema, now have methods that
indicate whether it's legal for one EntitySchema to migrate to another. This
logic is no longer in the SchemaManager.