crossbeam::skiplist: make iterator sendable to anther threads

[SpadeA-Tang]

We are currently implementing an in-memory engine feature in TiKV, and use skiplist as the underlying engine.
We need the iterator to be sendable among threads.
However, the current skiplist implementation carries the borrow of the skiplist, so the iterator can only be used in the thread creating it.
I implement a wrapper of the current skiplist, making it support creating iterators supporting the senario described above.
Would you mind reviewing it and seeing it is needed in crossbeam skiplist.

[SpadeA-Tang]

@taiki-e PTAL, thanks.