OPENDJ-1388 CR-3446 Implement simple changelog db based on single log file

[Note: actual merge of all org.opends.server.replication.server.changelog.file
 package content to be done in one shot in a future commit]

* Add new package org.opends.server.replication.server.changelog.file 
 containing the log file implementation 

* The new package contains the following classes :
** Classes implementing the log - independant from changelog and built from scratch
  - LogFile : implementation of log based on a single file
  - LogWriter : writer for the log
  - LogReaderPool : pool of readers for the log
  - Record : a (key, value) log record
  - RecordParser : interface for the convertion from record to bytes and bytes to record
  - DecodingException : exception thrown when record decoding fails
** Classes implementing the changelog API, based on the log implementation. 
 - FileChangelogDB
 - FileChangeNumberIndexDB
 - FileChangeNumberIndexDBCursor
 - FileReplicaDB
 - FileReplicaDBCursor
 - ReplicationEnvironment

* Add unit tests for org.opends.server.replication.server.changelog.file package

* Add new attribute 'ds-cfg-replication-db-implementation' in 
 ds-cfg-replication-server (ServerReplicationConfig class), allowing
 to choose the db implementation to use for changelog in directory server: 
 either je or log file. Default is 'je'.

* Add new option 'org.opends.test.replicationDbImpl' for test target in build.xml
 to choose the db implementation to for changelog when running the tests.
 Default is 'log' (log file).

* Update all replication unit tests to allow the selection of changelog db implementation
 to use in tests.
 
* Fix the ReferentialIntegerityPluginTestCase to do better cleanup at end of test in
 order to avoid side-effect on other tests.