db: add initial local database persistence integration

Ref: #76 (closed)

• Add primary key to schema. We can't call #save without a primary key defined in the table
• Add persist transaction step for Kaltura upload

This creates a MediaResources Sequel model that can be used in place of the previous Model struct to hold the metadata we need to persist locally for media records.

A persist step was added to facilitate this persistence more explicitly, and the persisted MediaResources instance has a to_kaltura method which can be called to serialize the data in a format expected for Kaltura upload.