minor update to benchmark script

Summary: Try to match some parameters from Dhruba's benchmarks on github

Test Plan: ran it

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D24687
convert Options from string

Summary: Allow accepting Options as a string of key/value pairs

Test Plan: unit test

Reviewers: yhchiang, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D24597