Replication importer should be able to resume an import

When importing documents using replication importer, problems can appears and we don't want to restart an import of the whole repository.
Using mbeans interface and importDocuments method, the importer is stating by cleanup the root folder before performing a brain new import.

The fix is:

• adding a new method to enable/disable an "resume" option
• when resume is on, the importer is testing and ignore it if the document already exist in the repository, import it if already imported.