Add RecordLinkageStrategy to configure the way record is compared with
candidates and which pairs are generated.
RecordLinkageSimpleStrategy and RecordLinkageBestStrategy are
implementation to be backward compliant.
RecordLinkageMixStrategy is a mix between simple and best : 
- calling registerMatch() with the best candidate if one record has prob
> config.getThreshold() and ignoring other
- or calling registerMatchPerhaps() for all records that are prob >
config.getMaybeThreshold()
- or calling registerNomatch() if no record is matching