Add a utility function to guess optimized options based on constraints

Summary:
Add a function GetOptions(), where based on four parameters users give: read/write amplification threshold, memory budget for mem tables and target DB size, it picks up a compaction style and parameters for them. Background threads are not touched yet.

One limit of this algorithm: since compression rate and key/value size are hard to predict, it's hard to predict level 0 file size from write buffer size. Simply make 1:1 ratio here.

Sample results: https://reviews.facebook.net/P477

Test Plan: Will add some a unit test where some sample scenarios are given and see they pick the results that make sense

Reviewers: yhchiang, dhruba, haobo, igor, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18741