vinyl: factor out vy_range_get_write_iterator()
vinyl: factor out vy_range_get_write_iterator()

All the three functions that need a write iterator, dump, compact, and
split, reuse the same code for initialization of the write iterator.
Factor it out into a separate function.

mejedi/tarantool,rtsisyk/tarantool,mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool