memtx: fix tuples references on concurrent replaces

Since 527b02a2 (memtx: add yields
during index build) memtx_build_on_replace was introduced to handle
concurrent updates. The problem here was that the tuples being handled
with this trigger did not get reference counter promotion, leading to a
number of wrong behavior cases. Now this problem is solved.
This problem was found through primary index altering with updates in
background fiber. Corresponding test is introduced.

Closes #4973