vinyl: create/delete write iterator in tx thread
vinyl: create/delete write iterator in tx thread

Soon, we will add a new in-memory index to a range not immediately on
dump, but lazily, on first transaction. This means range->mems list
won't be immutable any more from a worker thread's pov. Hence it will be
impossible to initialize the write iterator in task->execute() callback.
So move the write iterator initialization to the task constructor.

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