Boost access before mutex is unlocked

Summary:
This moves the use of versions_ to before the mutex is unlocked
to avoid a possible race.

Task ID: #

Blame Rev:

Test Plan:
make check

Revert Plan:

Database Impact:

Memcache Impact:

Other Notes:

EImportant:

- begin *PUBLIC* platform impact section -
Bugzilla: #
- end platform impact -

Reviewers: haobo, dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15279