OPENDJ-1902 (CR-6667) Remove Storage.getWriteableTransaction()

Thanks to Yannick who showed me the way with r12096, I finally found how to do it.

*Storage.java:
Removed getWriteableStorage() and close() methods.

Importer.java:
In IndexDBWriteTask:
- In ctor, accept a Storage rather than a WriteableTransaction + renamed txn field to storage and changed its type from WriteableTransaction to Storage.
- In call(), extracted all code to call0(WriteableTransaction) + now only open a WriteableTransaction and call call0(WriteableTransaction) + propagate WriteableTransaction down the method calls
- renamed flushMapToDB() to flushToDB() + renamed map parameter to idSets and its type from Map to Collection.
- (Unrelated to this change) renamed doRebuildIndexes() to rebuildIndexes0().

ReadableTransaction.java:
No longer extends Closeable becasue it is not needed anymore.