[DBClientConfig] Add a function to get set of issue tracker IDs
[DBClientAction] Move declaration of ActionUserIdSet to DBClientAction.cc

Because it's not required from other classes.
In addition, getActionUser() is renamed to ActionUserIdSet::get()
[DBClientAction] Move down the declaration of ActionUserIdSet to emphasize other important constants
[DBClientAction] Add a class "ActionValidator" to wrap checking owner ID

It will be also used for validating issue trackers of IssueSender actions.
[DBClientAction] Fix an inverted condition on deleting no owner actions
[DBClientAction] Add missing const qualifier
[DBClientAction] Check existence of incident tracker on getting/deleting IncidentSender actions
[DBClientAction] Rename functions to represent current behavior

deleteNoOwnerActions* ->
deleteInvalidActions
[testDBClientAction] Revert name of test_deleteInvalidAction to test_deleteNoOwnerAction

Because the old name represents the behavior correctly.
[testDBClientAction] Add test_deleteNoIncidentTrackerAction()
[testDBClientAction] Unify duplicated code
[testDBClientAction] Add test_getActionListWithNoIncidentTracker()
[testDBClientAction] Remove garbage code
[DBClientAction] Remove ActionValidator::noValid()

Because valid incident sender actions still may exist even if
user doesn't exist.
Merge pull request #338 from project-hatohol/delete-invalid-issue-senders

Delete invalid incident senders