use super_version in NewIterator() and MultiGet() function

Summary:
Use super_version insider NewIterator to avoid Ref() each component
separately under mutex
The new added bench shows NewIterator QPS increases from 515K to 719K
No meaningful improvement for multiget I guess due to its relatively small
cost comparing to 90 keys fetch in the test.

Test Plan: unit test and db_bench

Reviewers: igor, sdong

Reviewed By: igor

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D15609