Move the file copy out of the mutex.

Summary:
We now release the mutex before copying the files in the case
of the trivial move. This path does not use the compaction job.

Test Plan: DBTest.LevelCompactionThirdPath

Reviewers: yhchiang, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30381