vinyl: do not use restore callback for restarting read iterator

The read iterator has to restart (i.e. reopen all its sources) from the
position last returned to the caller when the current range or the whole
range tree changes as a result of dump or compaction. To reposition the
iterator, we use vy_stmt_iterator_iface->restore callback, which was
initially designed to restore an individual merge source (txw, mem, or
cache) after a statement is added to or removed from it. Abusing the
callback like that complicates its implementation as well as the read
iterator itself. We can avoid that by simply reopening merge sources
with the proper key when we need to restart the read iterator.