Fix HashSkipList and HashLinkedList SIGSEGV

Summary:
Original Summary:
Yesterday, @ljin and I were debugging various db_stress issues. We suspected one of them happens when we concurrently call NewIterator without prefix_seek on HashSkipList. This test demonstrates it.

Update:
Arena is not thread-safe!! When creating a new full iterator, we *have* to create a new arena, otherwise we're doomed.

Test Plan: SIGSEGV and assertion-throwing test now works!

Reviewers: ljin, haobo, sdong

Reviewed By: sdong

CC: leveldb, ljin

Differential Revision: https://reviews.facebook.net/D16857