Changed AttrHistorical.getValuesHistorical() return type to match usage


AttrHistorical.java:
In getValuesHistorical(), changed return type from Map<AttrValueHistorical, AttrValueHistorical> to Set<AttrValueHistorical> - to match the usage.

AttrHistoricalMultiple.java:
Consequence of the change to AttrHistorical.getValuesHistorical().
In delete(Attribute, CSN), now call delete(ByteString, CSN).
In add(Attribute, CSN), now call add(ByteString, CSN).
Extracted method update(CSN, AttrValueHistorical).
In conflictAdd(), changed return type to void since it was never used.
Removed javadoc copied from supertype + useless @inheritDoc.
Reduced method's visibilities.

AttrHistoricalSingle.java
Consequence of the change to AttrHistorical.getValuesHistorical().
Removed javadoc copied from supertype + useless @inheritDoc.
Used static import for HistAttrModificationKey.*.
Extracted method getSingleValue().

EntryHistorical.java:
Consequence of the change to AttrHistorical.getValuesHistorical().
Extracted method toOptionsString().
In newInstanceFromEntry(), reduced scope of variables.

AttrInfoTest.java:
Consequence of the change to AttrHistorical.getValuesHistorical().