Add container of operations on packages by their NEVRAs.
Remove unneeded sack parameter from open_history.
Return history of a transaction in the new container.
refactor: Rename functions in Base.history_undo.
refactor: Move obsoleted packages handling within Base.history_undo.
refactor: Move the content of HistoryCommand._hcmd_undo to BaseCli.history_undo_transaction.
refactor: Rename Base.history_undo to Base.history_undo_operations.
refactor: Move the content of HistoryCommand._hcmd_rollback to BaseCli.history_rollback_transaction.
Fix AttributeError raised when BaseCli.history_rollback_transaction called.
Add history-rollback-specific transaction check error output.
remove: force from history rollback command.
Enable the "history rollback" command (RhBug:991038)
doc: Use imperative mood in documentation of the undo command.
tests: Remove unneeded test of HistoryCommand.doCheck.
history: fix tests in py3.
remove: conf.config_file_age and repo.repo_config_age.

Don't play the config-file-age charade, the values are not used.
doc: api: Base.read_all_repos()
fixed JSON decode error in persistor.py (RhBug:1032455)
rename: Base.update*() -> Base.upgrade*()

To have the API sync with the preferred nomenclature.
doc: api: document marking packages for transactions in Base.