oaiharvester: OAI post process update

* Improves the logic and documentation of the tasks to
  post-process harvested records to be less error prone.

* Updates some tests with comments and missing logic.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
OAIHarvest: logging creation fix

* Properly sets the OaiHARVESTLOG.date_harvested value when creating
  a log entry by calling datetime.now() in the proper way.

* Removes exception masking in create_oaiharvest_log_str().

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
oaiharvester: record extraction improvement

* Improves the OAI-PMH record extraction to better use namespaces to
  output cleaner data without dummy namespaces.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
oaiharvester: post process check record

* Adds a new task check_record in the default post_process workflow
  to check if there is valid data in the record after conversion. If
  not, then skip further execution of this object.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
oaiharvester: configurable namespace addition

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
OAIHarvest: session_manager usage

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>