Set LastSuccessSynchronization timetamp when synchronize groups

 - new attribute group_def_lastSuccessSynchronizationTimestamp which is
   set only when exception message is null (it means no exception in the
   middle of synchronization)
 - use null value (attribute is not set) in lastSynchronizationState
   in meaning "no exception thrown"
 - if attrDef for new lastSuccessSynchronzationTimestamp not exists, only
   log this information to error log

There are 3 possible variants of behavior now:
 1] STATE = null, successTimestamp=null => not synchronized yet
 2] STATE = null, successTimestamp=NOTnull => last synchronization is OK
 without exceptions
 3] STATE = notNULL, lastTimestamp(not success one)=notNULL => last
 synchronization failed (or skipped some members)

 NEED TO BE DONE BEFORE MERGE:
  - should create new attribute:
   - value:String, group_def_lastSuccessSynchronizationTimestamp
  - gui must be able to work with this new workflow of null value for
    attribute lastSynchronizationState like in variants above