Checkpoint commit for OPENDJ-1206 : Create a new ReplicationBackend/ChangelogBackend 
 to support cn=changelog
CR-4053

Preparatory work for the changelog backend:  add a new behavior to cursors, in order to be
 able to start cursor at a given key instead of starting just after. 
 In order to avoid introducing another boolean in the methods, I created two enums to 
 define the behavior of a cursor.

Note that JE implementation does not yet implement the new behavior.

* DBCursor class : add two enums KeyMatchingStrategy and PositionStrategy
** The new behavior corresponds to PositionStrategy.ON_MATCHING_KEY, which allow 
 to position on the record with the given key (while AFTER_MATCHING_KEY position 
 just after the record with the given key).

* ReplicationDomainDB : add PositionStrategy argument for all methods that returns a cursor

* ReplicationServerDomain : getCursorFrom(DN, ServerState) method calls underlying method
  with PositionStrategy.AFTER_MATCHING_KEY

* FileChangelogDB : add PositionStrategy argument for all methods that returns a cursor

* FileReplicaDB : add PositionStrategy argument to generateCursorFrom(CSN) method

* FileReplicaDBCursor : add PositionStrategy argument to the constructor, implement the
  new behavior when position strategy is ON_MATCHING_KEY

* DomainDBCursor, MultiDomainDBCursor : add PositionStrategy argument to the constructor, 
 pass the strategy to underlying cursors

* Log, LogFile, BlockLogReader : implement the new behavior when position strategy 
  is ON_MATCHING_KEY

* ChangeNumberIndexer : use AFTER_MATCHING_KEY strategy when retrieving the 
  cursor (no behavior change)

* JEChangelogDB : add PositionStrategy argument for all methods that returns a cursor, 
   but getCursorFrom(DN, int, CSN, PositionStrategy) method does NOT implement the
   new ON_MATCHING_KEY strategy.

* Update of tests classes to match method signature but no new tests added for the 
  new behavior (ON_MATCHING_KEY) - to be done later