Implement a way to move a set of entries of one lexicon to another lexicon, within the same db instance.
A first implementation only allows entries of words unique to the lexicon from which they are moved (since merging entries from different lexica is non-trivial).
This should work by reassign the lexicon-ID of entries unique to the lexicon from which they are moved.

See T133859

(A different approach is to use insert/update/delete calls for one entry at a time.)