Summary:
This patch adds GetStringFromColumnFamilyOptions(), the inverse function
of the existing GetColumnFamilyOptionsFromString(), and improves
the implementation of GetColumnFamilyOptionsFromString().

Test Plan: Add a test in options_test.cc

Reviewers: igor, sdong, anthony, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: noetzli, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D45009