vinyl: fix assertion failure in vy_tx_handle_deferred_delete

vy_tx_handle_deferred_delete() expects (righteously) that in case a
deferred DELETE overwrites a statement in a secondary index write set
and the overwritten statement wasn't skipped on commit (i.e. doesn't
have txv->is_overwritten flag set), both the old and the new statement
must be REPLACEs (see the comment to the corresponding assertions for
more details).

The problem is we don't set is_overwritten flag in case a statement
doesn't have any effect (txv->is_nop is set), even if it was, in fact,
overwritten in the primary index write set (see vy_tx_prepare). As
a result, we get an assertion failure when we delete such statement
in the same transaction, e.g.

  s = box.schema.space.create('test', {engine = 'vinyl'})
  s:create_index('pk', {parts = {1, 'unsigned'}})
  s:create_index('sk', {parts = {2, 'unsigned'}})
  s:replace{1, 1, 1}
  box.begin()
  s:update(1, {{'+', 3, 1}}) -- adds no-op REPLACE to sk write set
  s:delete(1)                -- produces deferred DELETE for sk
  box.commit()

results in

  vy_tx_handle_deferred_delete: Assertion `vy_stmt_type(stmt) == IPROTO_REPLACE' failed.

Fix this by making sure we set is_overwritten for all overwritten
statements in a secondary index write set.

Closes #4294