OPENDJ-2016 Implement new on disk merge import strategy based on storage engine

OnDiskMergeStorageImporter now writes all keys/values to the Importer.
Added various toKey() and toValue() methods to the index classes.


OnDiskMergeStorageImporter.java
Write all keys/values to the Importer.
Rewrote the code inherited from OnDiskMergeBufferImporter + simplified the code


DN2ID.java:
Renamed dnToKey() to toKey().

DN2URI.java:
Added toKey() and toValue().
Used try-with-resources + diamond operator.

VLVIndex.java:
Extracted toKey() from encodeVLVKey().
Used try-with-resources.


VerifyJob.java:
Consequence of the change to VLVIndex.encodeKey().
Code cleanup.

IndexBuffer.java:
Code cleanup