CRM-15713 - CRM_Case_BAO_Case::accessCase - Split off from getCases().

This allows us to explicit control over more parameters (eg case_status_id)
and to improve performance (by focusing on the specific $caseId).
Merge pull request #4730 from totten/4.4-CRM-15713

CRM-15713 - CRM_Case_BAO_Case::accessCase - Split off from getCases().