Refactoring: created OAI component class.

refs #5023

Creation component class with OAI-PMH-specific helper methods.
Refactored OAI-PMH listIdentifiers response.

refs #5023

Refactored OAI-PMH listIdentifiers response to use OAI-specific
component class.
Refactored OAI-PMH listIdentifiers response.

refs #5023

Refactored OAI-PMH listRecords response to use OAI-specific
component class.