ComputeCompactionScore in CompactionPicker

Summary:
As it turns out, we need the call to ComputeCompactionScore (previously: Finalize) in CompactionPicker.

The issue caused a deadlock in db_stress: http://ci-builds.fb.com/job/rocksdb_crashtest/290/console

The last two lines before a deadlock were:
2014/03/18-22:43:41.481029 7facafbee700 (Original Log Time 2014/03/18-22:43:41.480989) Compaction nothing to do
2014/03/18-22:43:41.481041 7faccf7fc700 wait for fewer level0 files...

"Compaction nothing to do" and other thread waiting for fewer level0 files. Hm hm.

I moved the pre-sorting to SaveTo, which should fix both the original and the new issue.

Test Plan: make check for now, will run db_stress in jenkins

Reviewers: dhruba, haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17037