Implement Data Source

[ondrej-bouda]

• Introduce DataSource - an IRelationDefinition paired with a database connection, ready to be executed.
• Useful for a higher level of the application.
• Both having explicit methods like execute() or query(), and directly iterable, executing the query in the background.
• Occurrences:
  • IStatementExecution::dataSource() + implementations
  • IRelationDefinition::toDataSource(IConnection)
  • DataSource::fromRelationDefinition(IRelationDefinition, IConnection)
• Consider specifying a smaller part of IConnection to be held by the data source, just the necessary methods for a relation definition to be executable.
• Also consider an analogy for Command.