Code cleanup

IndexInputBuffer.java:
Simplified API:
- removed getKeyLen(), ensureRecordFetched() and currentRecord()
- renamed getIndexID() to currentRecord() + removed fetchKey()
- made public methods package protected

Importer.java:
Changed call0() return type to void.
Removed useless javadocs.
In IndexDBWriteTask:
- in call(), removed the ByteStringBuilder key
- consequence of the changes to IndexInputBuffer
- changed newImportIDSet(ByteStringBuilder key, Integer indexID) to newImportIDSet(ImportRecord).