OPENDJ-1707 Persistit: various import problems

Switched the import to use on-disk merge.
Remains to fix the multithreading issue (threadCount is currently hardcoded to 1).


BackendImpl.java:
In importLDIF(), do not forget to close the RootContainer.
Moved import preliminary code here from RootContainer.

Importer.java:
In processImport(), did the same fix as in r11932: split processImport() into several transactions to do commits after each step and expose data to all threads for the final on-disk merge.
In TmpEnv constructor expanded the scope of the proxy.

RootContainer.java:
In importLDIF(), extracted method importLDIFWithSuccessiveAdds() + added importLDIFWithOnDiskMerge() to offer several import strategies.
Import initialization code has been moved to BackendImpl.importLDIF().