Code cleanup


AttributeIndex.java:
Extracted method indexEntry()

Importer.java:
Extracted method indexEntry()
AppendReplaceTask:
- removed insertKeySet and deleteKeySet fields, superseded by ImportTask.insertKeySet field
ImportTask:
- renamed insertKeySet field to keySet
- in processIndexes(), merged AppendReplaceTask.processAllIndexes() + inlined fillIndexKey() and processAttribute()
- extracted method processAttribute0() out of processAttribute()

TestBackendImpl.java:
In assertIndexContainsID(), delegate to the other assertIndexContainsID() method.
AutoRefactor'ed file