vinyl: don't produce deferred DELETE on commit if key isn't updated

Consider the following example:

  s = box.schema.space.create('test', {engine = 'vinyl'})
  s:create_index('primary')
  s:create_index('secondary', {parts = {2, 'unsigned'}})

  s:insert{1, 1, 1}
  s:replace{1, 1, 2}

When REPLACE{1,1} is committed to the secondary index, the overwritten
tuple, i.e. INSERT{1,1}, is found in the primary index memory, and so
deferred DELETE{1,1} is generated right away and committed along with
REPLACE{1,1}. However, there's no need to commit anything to the
secondary index in this case, because its key isn't updated. Apart from
eating memory and loading disk, this also breaks index stats, as vy_tx
implementation doesn't expect two statements committed for the same key
in a single transaction.

Fix this by checking if there's a statement in the log for the deleted
key and if there's skipping them both as we do in the regular case, see
the comment in vy_tx_set.

Closes #3693