Introduce and use NoCrossReferencingLexemeStatements
The previous implementation of merge validation would not
catch references between the two different types of sub entity
(form and sense).
This new attempt has come from discussion on and about Iaf0582870
which was created to add Sense support for merging Lexeme entities.
The new approach brings the validation up to the Lexeme level
rather than doing it per sub entity type used during the merge.
T198106 is the origional ticket with the origional BDD for this
decision. It has also been double checked with Lydia and the folks that
worked on NoCrossReferencingFormStatements that I am removing here.
This patch also introduced LexemeStatementEntityReferenceExtractor.