Optimize usage of Status in CompactionJob

Summary: Based on @ljin feedback

Test Plan: compiles

Reviewers: ljin, yhchiang, sdong

Reviewed By: sdong

Subscribers: ljin, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28515
Bump Version Number to 3.8

Summary: As tittle.

Test Plan: Not needed.

Reviewers: ljin, igor, yhchiang, rven, fpi

Reviewed By: fpi

Subscribers: leveldb, fpi, dhruba

Differential Revision: https://reviews.facebook.net/D28629