use CAS when returning SuperVersion to ThreadLocal

Summary:
Add a check at the end of GetImpl to release SuperVersion if it becomes
obsolete. Also do Scrape() inside InstallSuperVersion so it happens more
frequent.

Test Plan:
make all check
running asan_check now

Reviewers: igor, haobo, sdong, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16641