add range scan test to benchmark script

Summary: as title

Test Plan: ran it

Reviewers: yhchiang, igor, sdong, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D25563
VersionBuilder to use unordered set and map to store added and deleted files

Summary: Set operations in VerisonBuilder is shown as a performance bottleneck of restarting DB when there are lots of files. Make both of added_files and deleted_files use unordered set or map. Only when adding the files, sort the added files.

Test Plan: make all check

Reviewers: yhchiang, rven, igor

Reviewed By: igor

Subscribers: hermanlee4, leveldb, dhruba, ljin

Differential Revision: https://reviews.facebook.net/D30051