using thread local SuperVersion for NewIterator

Summary:
Similar to GetImp(), use SuperVersion from thread local instead of acquriing mutex.
I don't expect this change will make a dent on NewIterator() performance
because the bottleneck seems to be on the rest part of the API

Test Plan:
make asan_check
will post perf numbers

Reviewers: haobo, igor, sdong, dhruba, yhchiang

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17643