New ExtSourceApi and optimization for extSources

 - rename old ExtSourceApi to ExtSourceSimpleApi
 - rename old methods findSubjects to findSubjectsLogins and change
   javadoc for these methods
 - create new ExtSourceApi which extends ExtSourceApiImpl and define new
   methods findSubjects
 - all extSources implements ExtSourceSimpleApi except XMLExtSource
   which use ExtSourceApi because of better optimization when using
   method findSubjects instead of findSubjectsLogins
 - create new method getCandidate from extSource which need only
   prepared subject Data in Map structure and can create member without
   calling extSource again (in entry, api, bl and blimpl)
 - in method findCandidates change behavior depending on used extSource
   (simple or classic)
  - if ExtSourceSimple is used: use findSubjectsLogins method and for
    every subject get attributes by method getCandidate from extSource
  - if ExtSource is used: use findSubjects method and for every subject
    get attribtues by method getCandidate from subjectData (map with
    attributes)
  - for both now also test uniqness of logins (throw exception if some
    login is there more than once)