Never CompactRange to level 0 in level compaction

Summary: I was bit by this when developing SpatialDB. In case all files are at level 0, CompactRange() will output the compacted files to level 0. This is not ideal, since read amp. is much better at level 1 and higher.

Test Plan: Compacted data in SpatialDB, read manifest using ldb, verified that files are now at level 1 instead of 0.

Reviewers: sdong, ljin, yhchiang, dhruba

Reviewed By: dhruba

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D20901
FeatureSet DebugString

Summary: This will help debugging

Test Plan: ran, observed output

Reviewers: yinwang

Reviewed By: yinwang

Differential Revision: https://reviews.facebook.net/D20937