vinyl: clean up write iterator source destruction

By convention we have two methods in each write iterator stream
implementation (including the write iterator itself as it implements
the interface too): 'stop' and 'close'. The 'stop' method is called
in a worker thread. It reverses the effect of 'start'. We need it
unreference all tuples referenced during the iteration (we must do
it in the worker thread, where the tuples were referenced in the first
place so as not to unreference tuple formats, see vy_tuple_delete).
The 'close' method is called from the tx thread to unreference tuple
formats if necessary and release memory.

For the write iterator itself we follow this convention. However,
for individual sources, for vy_slice_stream source to be more exact,
we do not - the write iterator calls both 'stop' and 'close' from
its own 'stop method. Let's cleanup this mess and make the write
iterator follow the convention. We'll need it in the next patch.