OPENDJ-1389 – Add support for replication changelog DB rotation

Implemented a log based on multiple log files 

* Add Log class that manage a log as a set of log files:
** it contains at least one log file, the head log file, 
     where new records are appended
** it contains from zero to multiple read-only log files, 
    issued from rotation of the head log file when it reaches
    a given size
* Update LogFile class to act as part of a Log.
* Add purging feature
* Update other classes from file package with minor changes
* Add unit tests for new Log class and purging feature

* Update MeteredStream class to use it in changelog
* Add new interface RotatableLogFile (to be used later by changelog)
* Update Policy classes to use this new interface