Change default value of some Options

Summary: Since we are optimizing for server workloads, some default values are not optimized any more. We change some of those values that I feel it's less prone to regression bugs.

Test Plan: make all check

Reviewers: dhruba, haobo, ljin, igor, yhchiang

Reviewed By: igor

CC: leveldb, MarkCallaghan

Differential Revision: https://reviews.facebook.net/D16995