memtx: skip excluded tuples in index count with MVCC enabled

Excluded tuples actually have their own history chains in MVCC - such
chains consist of only one `memtx_story` containing excluded tuple
itself. Such chains should be skipped when counting invisible tuples
because they are not inserted to the index - that's what the commit
does.

Closes #10396

NO_DOC=bugfix

(cherry picked from commit 8947cb04f59423e2944d48b8a1effec2fb11b1db)