comments about the BlockBasedTableOptions migration in Options

Summary: as title

Test Plan: none

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22737
update HISTORY.md

Summary: as title

Test Plan: no

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22761
delete unused Comparator
Merge pull request #263 from wankai/master

delete unused Comparator