avoid shared_ptr assignment in Version::Get()

Summary:
This is a 500ns operation while the whole Get() call takes only a few
micro!

Test Plan: ran db_bench, for a DB with 50M keys, QPS jumps from 5.2M/s to 7.2M/s

Reviewers: haobo, igor, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17007