vinyl: decompose read iterator merge procedure

The generic approach trying to build the merge procedure around the
vy_stmt_iterator interface didn't pan out, because sources are way too
different: in contrast to other sources, the cache stores intervals;
run iterators may yield; txw does not preserve statement history.

Let's rewrite vy_read_iterator_next_{key,lsn} in such a way that they
do not use this generic interface. This results in a quite bit of code
being duplicated, because loops over sources are unrolled, but this is
intentional - hopefully it makes the code easier to follow. The patch
isn't supposed to change the merge algorithm or remove any optimization
implemented in it.