vinyl: fix crash in vy_read_iterator_evaluate_src

There's an optimization in vy_read_iterator_evaluate_src(): in case
the evaluated statement exactly matches the search key, we assume
that it precedes all statements that have been evaluated so far.
This assumption holds when we are looking for the next key, but it
results in a crash when we are applying UPSERTs:

  vy_read_iterator_evaluate_src: Assertion `vy_read_iterator_cmp_stmt(itr, src->stmt, itr->curr_stmt) < 0' failed

Actually, this optimization is rather dubious. It saves us one
comparison on the first iteration if the iterator type is LE/GE, the
search key is full, and there's an exact match. However, if there's
no exact match, it will add an extra comparison instead. One might
argue that we should optimize for exact match because of space.get(),
but for space.get(), the read iterator is not invoked at all - we use
point lookup instead. That being said, let's simply zap this
"optimization" altogether to fix this crash.

Closes #3003