vinyl: make read iterator always return newest tuple version

vy_read_iterator_next() first scans all sources to find the newest
statement for the next key. If the statement turns out to be UPSERT,
it retrieves older statements until a terminal statement is found.
While retrieving older statements, it may yield, which opens a window
for other fibers to insert newer statements for the same key. Those
statements will be ignored by the read iterator even if they are
visible from the iterator read view.

To be able to use read iterator for building secondary indexes (i.e.
iterate over tuples stored in pk and insert them into the new index),
we need to lift that limitation, otherwise there's a good chance that
while building a new index we will overwrite statements inserted by
concurrent transactions. So this patch reworks the read iterator to
guarantee that it always returns the newest tuple version.

To achieve that, we now retrieve the whole key history instead of
only the last statement first time we scan a source. Respectively,
we store a vy_history instead of tuple in vy_read_src. All source
iterators now return vy_history too. As a result, we don't need to
iterate to the next LSN (and possibly yield) to apply UPSERTs - we
simply splice all histories corresponding to the next key and call
vy_history_apply() to return the resultant tuple.

Note, this patch changes the output of vinyl/upsert.test.lua. This
is OK as now UPSERTs are applied in the reverse order, from the
oldest statement to the newest.