make hash_link_list Node's key space consecutively followed at the end

Summary: per sdong's request, this will help processor prefetch on n->key case.

Test Plan: make all check

Reviewers: sdong, haobo, igor

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17415
Create log::Writer out of DB Mutex

Summary: Our measurement shows that sometimes new log::Write's constructor can take hundreds of milliseconds. It's unclear why but just simply move it out of DB mutex.

Test Plan: make all check

Reviewers: haobo, ljin, igor

Reviewed By: haobo

CC: nkg-, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D17487
Remove -Wno-unused-const-variable
relax backupable db rate limit tests
Update HISTORY.md

Summary: Update HISTORY.md to make existing items to 2.8 release and add something I think is missing.

Test Plan: N/A

Reviewers: haobo, igor, ljin, dhruba, yhchiang, xjin

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17517